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Chapter One: Road Design Manual Introduction

1.1 Purpose The Delaware Department of Transportation (DelDOT) has developed the Road Design Manual to provide guidance and assistance in the standard practice of design of roadway features in the state of Delaware. The Road Design Manual adopts industry recognized design criteria and standards or prescribes Delaware specific criteria and standards for design of roadway features. Additionally, the Road Design Manual provides a general discussion of the relationship between design controls/criteria and design standards to provide context for various design elements.

1.2 State and Federal Laws and Regulations Title 17 of Delaware Code grants to DelDOT the absolute care, management and control of all public roads and highways in the State of Delaware, excluding a small number of roads in incorporated municipalities. Title 23 Highways of the Code of Federal Regulations (23 CFR) and more specifically 23 CFR Part 625-Design Standards for Highways (23 CFR Part 625) grants the authority to the Federal Highway Administration (FHWA) in cooperation with State transportation departments, to develop criteria to implement design for the National Highway System (NHS). Delaware applies these standards uniformly for all roads including non-NHS roads. For specifics regarding FHWA’s role in transportation projects in Delaware, reference shall be made to the most current Stewardship and Oversight Agreement on Project Assumption and Program Oversight between FHWA, Delaware Division and DelDOT

1.3 Primary Source for Design Criteria and Standards The American Association of State Highway and Transportation Officials (AASHTO) is the recognized authority on highway design policies and standards. With the exception of certain DelDOT specific design criteria and standards presented in subsequent chapters of this manual, the American Association of State Highway and Transportation Officials 2018 7th edition of A Policy on Geometric Design of Highways and Streets (AASHTO Green Book) is adopted and recognized as the source for design criteria and standards used for all DelDOT highway projects. All DelDOT specific design criteria and standards appearing in subsequent chapters of this manual are presented in bold text.

1.4 Additional Reference Documents In addition to the AASHTO Green Book this manual relies on many other federal and state resource documents and manuals that expand upon the information in the AASHTO Green Book when determining design criteria and standards for use on DelDOT transportation projects. All DelDOT authored documents referenced here are the latest edition. These reference documents appear in italics in subsequent chapters of this manual and include but are not limited to: • AASHTO Roadside Design Guide 2011 4th Edition • AASHTO Policy on Geometric Design Standards - Interstate System 2016 6th Edition • AASHTO Highway Safety Manual 2010 1st Edition • AASHTO Drainage Design Manual • AASHTO Guide for the Planning, Design and Operation of Pedestrian Facilities • AASHTO Guide for the Development of Bicycle Facilities • Hydraulic Engineering Circular No. 22 - Urban Drainage Design Manual, 3rd Edition • Hydraulic Engineering Circular No. 17 - Highways in the River Environment • Hydraulic Engineering Circular No. 25 – Highways in the Coastal Environment • Transportation Research Board Highway Capacity Manual 2016 6th Edition • DelDOT Project Development Manual • DelDOT Pedestrian Accessibility Standards for Facilities in the Public Right of Way 2021 Edition • Delaware MUTCD • DelDOT Standard Construction Details • DelDOT Standard Specifications • DelDOT Development Coordination Manual • DelDOT Traffic Design Manual • Delaware Transit Corporation’s Bus Stop and Passenger Facilities Policy, Revised 2018 • NCHRP Report 672, Roundabouts: An Informational Guide, 2nd Edition • NCHRP Report 600, Human Factors Guidelines for Road Systems, 2nd Edition

1.5 Process Related Material and Guidance Process related material used in the development of DelDOT transportation projects can be found in other sources including the DelDOT Project Development Manual, the DelDOT Design Resource Center, and in DelDOT Design Guidance Memoranda. DelDOT’s Project Development Manual describes processes to develop and design DelDOT transportation projects. Its main focus is the documentation of DelDOT’s plan development process and coordination between sections within DelDOT and outside agencies required to design and implement a transportation project. In addition, the Design Resource Center contains project development guidelines and resources to be used during the preparation of design plans to insure consistency in the plan development process.

Chapter Two: Design Controls and Criteria

2.1 Introduction Plans for transportation improvement projects are based on established geometric design standards for the various elements that constitute a roadway. Decisions on appropriate geometric design standards are influenced by the type of project and the project area. Each project type and area has its own unique characteristics. These characteristics need to be considered when determining a projects design controls and criteria which dictate the design standards used when designing a project. These design controls include design speed, traffic volumes, design vehicles, functional classification, design level of service and project type.

2.2 Design Speed Design speed has a greater influence on the design of a project than any other design control. To some degree design speed impacts all elements of a projects geometric design standards. It is important to note that design speed is a selection and the selected design speed should be logical with respect to the characteristics of the terrain, adjacent land use, traffic volume and functional classification of the roadway. Selecting a design speed equal to the anticipated posted speed has become an increasingly common practice in many agencies and is the method of design speed selection for DelDOT projects. In many cases evaluating operating speeds through the use of speed studies is an important tool when selecting the most appropriate design speed.

2.3 Traffic Volumes The existing and future expected motor vehicle traffic volume is a significant factor in establishing a project design. Some important traffic volumes for design purposes include Annualized Average Daily Traffic (AADT), Design Hourly Volume (DHV), and Directional Distribution (D). For all projects involving new construction or major reconstruction, the design controls normally will be based on the traffic volumes estimated for 20 years after the expected completion of the project, expressed as AADT and DHV. For projects where the scope of work is limited to minor modifications to the existing roadway network, traffic volumes estimated for 10 years after the expected completion of the project should be used. Some asset management projects such as pavement rehabilitation projects will replace in-kind and/or make very minor modifications to improve traffic flow and/or safety within their limited scope of work, in which case traffic projections and analysis may not be required at all. Existing and future expected pedestrian volumes and cyclist traffic volumes must be considered as well. For projects on existing facilities, typically pedestrian and cyclist volumes will be counted. However, there are no good quantitative procedures available to predict future pedestrian and cyclist volumes. For most projects, if there is any expected level of pedestrian or cyclist demand, the design should include facilities to accommodate that demand. In particular, pedestrian desire lines should be considered related to sidewalks, multi-use paths, and crosswalk locations. Some smaller projects with limited scope of work may not be able to add these multi-modal facilities.

2.4 Design Vehicles Where turning movements are involved, the geometric design requirements are affected significantly by the types of vehicles using the facility. Four general classes of vehicles are identified including passenger cars, buses, trucks, and recreational vehicles. The passenger car class includes cars of all sizes, sport utility vehicles, vans, and pick up trucks. Buses include school buses, city transit buses and motor coaches. The truck class includes single unit trucks and truck and trailer combinations. Recreational vehicles include motor homes and cars or motor homes pulling camper trailers or boat trailers. Each design vehicle has physical and operational characteristics that affect the design controls including acceleration and deceleration capabilities, ability to climb steep grades, and sweep path dimensions of turning vehicles.

2.5 Functional Classification DelDOT has adopted a system of classifying and grouping highways, roads, and streets based on FHWA guidance found in the latest version of Highway Functional Classification Concepts, Criteria and Procedures. These recognized functional classifications include Interstate Highways, Freeways and Expressways, Principal and Minor Arterials, Major and Minor Collectors and Local Roads and Streets. These functional classifications are further broken down into urban and rural classifications depending on the roadways setting. Determining the appropriate functional classification is critical when defining a transportation projects design standards.

2.6 Level of Service Level of service characterizes the operating conditions on a facility in terms of traffic performance measures such as delay, speed, and density. It establishes a grading system, where letter grades from A through F are assigned to different types of highway facilities based on their traffic performance. The Transportation Research Board’s (TRB) Highway Capacity Manual (HCM) presents a thorough discussion of the concept of level of service, specific methods to estimate traffic performance measures, and charts that establish level of service based on those traffic performance measures. In addition to the specific traffic performance measures that are directly related to level of service, there are other important traffic performance measures that can be measured and/or estimated such as travel time delay, queuing, and pedestrian level of stress. Furthermore, beyond the estimating methods presented in the HCM, there are other methods which may be used, in particular traffic simulation models. Level of service is closely related to the concept of capacity. Capacity is the maximum traffic flow that can be accommodated in a highway facility during a given time period under prevailing roadway, traffic and control conditions. Generally, the dividing line between Level of Service E and F coincides with a volume-to-capacity (v/c) ratio of 1.0. In other words, when the v/c ratio is less than 1.0, the LOS is E or better, and when the v/c ratio is greater than 1.0, the LOS is F. While the HCM provides methods to determine LOS for motor vehicles, cyclists, and pedestrians, for transportation projects in Delaware the LOS (and/or other traffic performance measures) related to motor vehicle traffic are nearly always the controlling factor related to a project design. Including multi-modal facilities within a transportation project are critical, but the design of the multi-modals aspects of a project do not rely on multi-modal LOS. DelDOT projects should generally strive to attain a LOS of E or better based on design year traffic projections and analyses during the design hours. This may vary based on the context of the project, or other factors such as being included within a Transportation Improvement District (TID) which has a previously defined service level. The design hours are often peak traffic hours related to morning and afternoon commuting periods, but could also include weekend shopping peaks, summer recreational peaks, school peaks, or others depending on the context of the project.

2.7 Project Type There are many types of projects requiring varying levels of design effort and selection of design controls and criteria. Occasionally projects involve new alignments and new construction or major reconstruction that require significant changes in grades and geometry. New construction and reconstruction projects should be in conformance with appropriate design criteria and standards and exceptions should be rare. Many projects are planned and designed to maintain the existing highway system and many projects are funded to address immediate needs such as improvement of the riding service and improving traffic services and safety. These projects may include pavement overlays, widening, flattening of slopes, alignment or grade changes, traffic control device improvements, drainage improvements, curb and gutter and channelization. These projects may also include the addition of facilities to better serve bicycles, pedestrians, and transit. For these type projects the proposed scope of work, available funding and project needs must be evaluated when establishing the design criteria and standards.

Chapter Three: Design Standards

3.1 Introduction Designers are called upon to make numerous decisions as to the geometrics and physical characteristics of transportation improvement projects. Without some basic framework of design standards, the judgements of individual designers may vary considerably. The purpose of design standards is to assure that transportation improvement projects are consistently designed with due consideration of appropriate levels of service, safety, and economy consistent with the environmental and social context of the area and driver expectancy. Designers should be aware that there is flexibility in the standards set forth by AASHTO that allows choices to be made as the design progresses and complex community and environmental issues arise. Since there are many decisions made during the design process affecting the standards, documentation of these decisions is a critical part of the design. Designers shall document the projects standards on DelDOT specific design checklists located on the Design Resource Center.

3.2 Departure from Design Standards New construction or reconstruction projects on roadways should meet all applicable standards. The need for exceptions to the standards should be identified early in the project development process in order that approvals or denials will not delay the completion of the design. DelDOT’s Design Criteria Form and Design Control Checklist should be used to document decisions on design standards and as a basis for developing and documenting requests for design exceptions. The FHWA has established controlling criteria for design, as listed on the Design Criteria Form, which when not met require design exceptions. An exception for design speed should not be sought as this criteria establishes most of the design standards to be met. Design standards that cannot be met within a selected design speed should be supported by seeking a design exception.

3.3 Standards Based on Design Speed Determining the appropriate design speed is the first step in establishing several design standards that are directly related to the design of a transportation facility and is discussed previously in this manual and at length in the AASHTO Green Book. Some design features, such as curvature, superelevation, and sight distance are directly related to, and vary appreciably with, design speed. Other features, such as width of lanes and shoulders and lateral clearances, are not directly related to design speed but do affect vehicle speeds.

3.3.1 Horizontal Alignment, Curvature and Superelevation Horizontal alignment of a roadway is defined graphically using a series of straight-line tangents with transition sections into and out of horizontal curves. When developing horizontal alignments designers should be aware that DelDOT no longer employs the use of spiral curves and horizontal alignments should be developed using simple curves only.

Establishing the proper relationship between design speed and curvature, as well as their joint relationship with the proper amount of superelevation on the curve is an important decision. Although these relationships are derived from laws of physics (speed, centrifugal force, and side friction factor), the actual values for use in design depend on practical limits and factors determined empirically over a range of variables. For example, the maximum permissible rate of superelevation is based on a practical consideration that a high operating speed can be accommodated on a relatively sharp curve if the superelevation is steep enough, however roadways must serve vehicles traveling at a wide range of speeds. Slow moving vehicles or stopped vehicles would be adversely affected with excessively steep superelevation, particularly in ice and snow conditions. AASHTO suggests maximum superelevation rates in the range of 4 to 12 percent. Delaware’s roadways are subject to the effects of ice and snow during the winter; therefore, DelDOT has adopted a maximum superelevation rate of 6 percent.

3.3.2 Sight Distance Sight distance is the length of roadway ahead of the vehicle that is visible to the driver. Horizontal curvature, vertical curvature, roadside obstructions, or any combination of these elements can restrict sight distance.

3.3.2.1 Stopping Sight Distance Stopping sight distance is the total distance to bring a vehicle to a complete stop and is affected by the time for a driver to react and the braking distance. The available stopping sight distance must always be sufficient to enable a vehicle traveling at the design speed to come to a stop before reaching an object on the roadway. Factors that influence the required stopping sight distance include the speed of the vehicle, the drivers eye height, the height of the object on the road, the driver’s reaction time and the surface condition of the road.

3.3.2.2 Passing Sight Distance Consideration of passing sight distance is limited to two-lane, two-way roadways on which vehicles frequently overtake slower moving vehicles and the passing operation must be accomplished within a lane used by opposing traffic. Passing sight distance for design is determined on the basis of the length needed to accomplish the passing maneuver. Passing sight distances for design should not be confused with other distances used as warrants for placing no-passing pavement markings on completed highways. Values shown in the Delaware MUTCD are substantially less than the design distances and are derived from traffic operation control needs based on assumptions different from those for design.

3.3.3 Vertical Alignment and Grades The topography of the land has a significant influence on the alignment of roadways. Topography affects horizontal alignment but has an even more pronounced effect on vertical alignment. Variations in topography are generally separated into three different classifications according to terrain; level, rolling, and mountainous. The terrain on which a roadway is located has a direct impact on the grades associated with the vertical alignment of the roadway. Maximum allowable grades are based on the functional classification of the roadway and the design vehicle anticipated to utilize the roadway and should only be used where absolutely necessary. Maximum grades permitted are based on various combinations of functional classification, design vehicle and terrain. Minimum allowable grades are based on the minimum slopes required to properly drain the pavement surface of the roadway. Due to very flat terrain in many areas in Delaware, minimum allowable grades of 0.3% may be used when developing vertical alignments.

3.4 Standards Based on Traffic Volumes and Design Vehicle Standards not directly related to design speed are influenced primarily by traffic volumes and the appropriate design vehicle.

3.4.1 Number of Lanes The number of travel lanes required for any roadway is directly related to the facility’s traffic volume and desired level of service; however, there is no simple fixed criteria for these relationships. The Highway Capacity Manual (HCM) provides one set of analysis methods for determining the number of travel lanes required to obtain a design flow rate and the analysis method varies based on the type of facility. Additional information is included in Section 2.6.

3.4.2 Travel Lane Widths The traveled way designated for vehicles normally consists of two or more travel lanes. The impact of providing adequate lane widths is wide ranging and includes maintaining and/or enhancing driver safety, driver comfort, level of service and capacity. For new construction and reconstruction projects, 12-foot travel lanes should be used on roadways with design speeds of 55 mph or greater and 11-foot travel lanes should be used on roadways with design speeds from 35 mph to 50 mph. 10-foot travel lanes should be used on roadways with design speeds below 35 mph provided they are not adjacent to bike lanes. 10-foot travel lanes should also be avoided along transit routes and roadways with heavy truck traffic.

3.4.3 Shoulder Widths The shoulder is the portion of the roadway contiguous with the traveled way that accommodates stopped vehicles, emergency use, bicycles, pedestrians, and provides lateral support to the traveled way of the subbase, base, and surface courses. The total shoulder width is the distance from the edge of travel lane to the intersection of the shoulder slope with the front slope, or the face of curb. For DelDOT projects, all interstate highways, freeways and expressways with high truck volumes the width of shoulder should be 10 feet. Outside shoulder widths of 5 to 8 feet should be used on all other new construction and reconstruction projects. Minimum shoulder widths of 5 feet should be used where bicyclists or pedestrians are present unless there is a separate shared use facility. In rural locations where reconstruction or repaving is proposed, designers should attempt to achieve a 2’ minimum shoulder width.

3.5 Cross Sectional Elements In addition to travel lane and shoulder width there are additional elements that make up a roadways cross section. The term cross section is used to define the configuration of a proposed roadway at right angles to the centerline. Typical sections show the width, thickness and description of the pavement section, as well as the geometrics of the graded roadbed, roadside ditches, side slopes and clear zones. The cross section elements of a facility are the most visible features to the driver and have the greatest physical effect on the construction of a roadway.

3.5.1 Clear Zone and Lateral Offset The clear zone is defined by the AASHTO Roadside Design Guide as the unobstructed, traversable area provided beyond the edge of the through traveled way for the recovery of errant vehicles. The clear zone includes shoulders, bike lanes and auxiliary lanes, except those that function like through lanes such as bypass lanes. Vehicles leaving the roadway should have a reasonable opportunity to recover control and return to the roadway without overturning or colliding with fixed roadside obstacles such as trees, utility poles, headwalls, or other large non-breakaway objects. The determination of the clear zone is a function of speed, volume, horizontal curvature, and embankment slope. The AASHTO Roadside Design Guide shall be used for determining appropriate clear zone widths and for all elements associated with designing an appropriate roadside environment. Where upright curb is used, the lateral offset is measured from the face of the curb to an object. This concept is used to identify acceptable locations for fixed objects in urban environments where upright curb is present. Reference shall be made to the AASHTO Roadside Design Guide for more information on this concept and its applications.

3.5.2 Cross Slopes It is important to enable surface water to drain from travel lanes and shoulders as quickly as possible. Accumulations of water (ponding) can cause hazards by reducing surface friction and vehicle stability. Sufficient cross slope is needed for adequate drainage, but too great a slope adversely affects vehicle operation. In addition, good drainage minimizes moisture penetration at the pavement/shoulder joint thus increasing stability of the mainline pavement. Sloping the shoulders steeper than the travel lanes assures rapid surface drainage, reduces the chance of ponding, and minimizes subgrade penetration of moisture through the edge joint.

3.5.3 Curbs Curbs generally serve the following purposes: drainage control, roadway edge delineation, right of way reduction, delineation of pedestrian walkways, and delineation of access points. Curb configurations include both vertical and sloping curbs. Vertical curbs are those having a vertical or nearly vertical face six inches or higher. They are intended to discourage motorists from deliberately leaving the roadway. Sloping curbs are those having a sloping traffic face six inches or less in height and can be readily traversed by motor vehicles when necessary. For DelDOT projects the use of vertical curb is limited to design speeds of 45mph and below. Curbs four inches and lower with a nearly vertical face and sloping curbs may be considered for use on facilities with design speeds above 45 mph when necessary due to drainage considerations, restricted right of way, or where needed for access control. When used under these circumstances, they should be located at the outside edge of the shoulder. DelDOT’s Standard Construction Details shall be referenced for the types of curbs used on projects in Delaware.

3.5.4 Side Slopes Side slopes are important in maintaining the stability of the roadbed and pavement structure as well as providing an area safe for errant vehicles. There is a distinct relationship between the steepness of side slopes and the required clear zone. Side slopes shall be evaluated and designed in accordance with the latest version of the AASHTO Roadside Design Guide.

3.5.5 Roadside Ditches The two principal functions of roadside ditches are to drain water from the pavement subgrade and to collect surface water either from the roadway surface or adjacent roadside areas and remove it before it enters the pavement subgrade. Ditch designs can play a major role in managing stormwater runoff, removal of sediment, controlling erosion, and reducing the impact of roadway pollutants on watercourses. Ditch cross sections should be traversable within the clear zone and designed in accordance with the criteria presented in the latest version of the AASHTO Roadside Design Guide.

3.5.6 Medians Medians are provided on divided multi-lane highways to provide separation of opposing travel lanes, a recovery area for errant vehicles and an area for emergency stops. Medians also provide space for left turn lanes, snow removal storage, space for collecting surface drainage, pedestrian refuge, installation of traffic control devices and adding future lanes. Median widths are always measured between the inside edges of opposing travel lanes and operate best when they are highly visible during the day or night. Reference shall be made to both the AASHTO Green Book and AASHTO Roadside Design Guide for design criteria and standards regarding medians.

3.5.7 Barriers The purpose of traffic barriers is to reduce fatalities and injuries by preventing a vehicle from leaving the traveled way and striking a fixed object or terrain feature that is less forgiving than striking the barrier itself. The need for barriers is directly related to the selected cross-sectional elements as well as the clear zone concept discussed previously. The need for barrier and its design shall be in accordance with the latest version of the AASHTO Roadside Design Guide. Roadside barriers are used to shield obstacles located along either side of the traveled way. W-beam steel guardrail and concrete safety barriers are the most commonly used roadside shielding devices. Geometric design criteria and typical installations are located in the DelDOT Standard Construction Details. The basic function of median barriers is to prevent errant vehicles from crossing the median and entering opposing travel lanes. Median barriers should be installed on all high-volume, high speed divided highways where engineering studies establish a need or in accordance with the latest version of the AASHTO Roadside Design Guide. The most common types of median barriers are W-beam steel guardrail, concrete safety barrier, and high tension cable barrier. Concrete safety barrier is preferred in narrow medians where regular maintenance is difficult, or where deflection of the barrier would affect opposing traffic. Vehicles impacting the untreated end of a roadside barrier can result in serious consequences. The vehicles may be stopped abruptly, barrier elements could penetrate the passenger compartment, or the vehicle may become unstable or potentially roll over. A wide variety of devices are considered end treatments and are designed to mitigate vehicle impact with ends of roadside barriers. The AASHTO Roadside Design Guide, DelDOT Standard Construction Details, and the DelDOT Standard Specifications shall all be referenced when considering and designing proper end treatments.

Chapter Four: Intersections

4.1 Introduction The intersection of two or more roads presents an opportunity for conflict among vehicles. For freeways the potential for conflict is significantly reduced by the use of interchanges or grade separated intersections which are typically not feasible for the vast majority of intersections on arterials and collectors. The principal objectives in the design of at-grade intersections are to minimize the potential for and severity of conflict points and to provide adequate mobility and capacity for the intersecting movements of all road users. Intersection designs range from simple residential driveways to a complicated convergence of several high-volume multi-lane roadways. Intersection designs also implement various types of traffic control including two-way stop controlled, all-way stop controlled, signalized and roundabouts. They all have the same fundamental design elements: level of service, alignment, profile, roadway cross section, intersection radii and sight distance. In addition, other elements are introduced in intersection designs such as turning lanes, auxiliary lanes, traffic islands, medians, channelization, and bicycle/pedestrian accommodations.

4.2 Turning Movements All intersections involve some degree of vehicular turning movements and there are various factors that influence the geometric design of turning lanes. The design controls for turning roadways are the traffic volume and types of vehicles making the turning movements. The movement of primary concern in intersection design is right turning traffic. The three typical types of designs for right turning roadways in intersections are a minimum edge of traveled-way design, a design with a corner triangular island, and a free flow design using a simple radius or compound radii. Designers should be aware that large right turn radii often required for larger design vehicles lead to high speed right turning traffic. These large radii are not desirable at locations with an appreciable amount of bicycles and pedestrians as they result in longer crossing distances for pedestrians and safety concerns for bicycles associated with the high speed right turning traffic. If a small right turning radius is selected to reduce turning vehicle speeds, a mountable outside apron may be required to accommodate larger design vehicles.

4.3 Channelization Channelization is the separation or regulation of conflicting traffic movements into definite paths of travel by traffic islands or pavement marking to facilitate the safe and orderly movement of vehicles, bicycles, and pedestrians. Proper channelization increases capacity, improves safety, and aids in driver navigation. Over channelization should be avoided because it could create confusion and operational problems within the intersection. Channelization of at-grade intersections is generally warranted for a number of factors including reducing pavement area to narrow the area of potential conflict between vehicles, providing clearer indication for the proper path in which movements are to be made, provide areas for pedestrian refuge, and areas for traffic control devices. Design of a channelized intersection usually involves the following controls – the type of design vehicle, the cross sections on the crossroads, the projected traffic volumes, the number of pedestrians, the speed of vehicles, and the type and location of traffic control devices. In addition, physical controls such as right-of-way and terrain have an effect on the extent of channelization that is economically feasible.

4.4 Intersection Sight Distance The operator of a vehicle approaching an at-grade intersection should have an unobstructed view of the whole intersection and of a sufficient length of the intersecting highway to permit control of the vehicle to avoid collisions, termed approach sight distance. The minimum sight distance considered safe under various assumptions of physical conditions and driver behavior is directly related to vehicle speeds and the resultant distances traversed during perception, reaction time, and braking. In addition to approach sight distance, sight distance is also provided to allow stopped vehicles sufficient view of the intersecting roadway to decide when to enter or cross the intersecting roadway, termed departure sight distance. Both sight distances must be checked on all intersection designs based on the procedures set forth in the AASHTO Green Book.

4.5 Auxiliary Lanes Auxiliary turning lanes may be introduced at intersections under a variety of conditions including rural or urban locations and free flowing, signalized or stop controlled facilities. Using auxiliary lanes to handle turning movements at high traffic volume intersections can reduce congestion, improve safety, and provide better traffic control. Auxiliary lanes are also used on multi-lane divided roadways and high volume-two lane roadways under open road conditions. They improve safety and traffic flow when introducing median openings, intersections at minor cross-roads or U-turn locations. Auxiliary lanes include left and right-turn deceleration lanes, right-turn and median acceleration lanes, and auxiliary through lanes. To facilitate the movement of traffic at signalized intersections efficiently, all mainline approaches should have at a minimum one dedicated left turn lane in addition to the through lane. Additionally, all side street approaches at signalized intersections should at a minimum have a two lane approach with either a shared left turn through lane configuration or a shared right turn through lane configuration as determined by traffic and turning movement analysis. At many high traffic volume intersections the use of auxiliary through lanes should be considered. Auxiliary through lanes function as additional through lanes on the approaches to the intersection that more efficiently move traffic through the intersection. The use of auxiliary through lanes require properly designed tapers for both the lane addition in advance of the intersection and the lane drop exiting the intersection. The length of auxiliary lanes depends on local conditions, traffic volumes, design speed, selected level of service, and operating speed. Ideally, turn lanes should be long enough to accommodate 95 percent of the expected queue for the design volume in the design year. Auxiliary lanes should be 11 feet wide to minimize encroachment of turning vehicles upon the adjacent travel way. A minimum 10 foot wide auxiliary lane may be acceptable in locations where space is limited and operating speeds are low. Acceleration lanes are not desirable where the requisite acceleration lane length can not be achieved or where entering drivers can wait for an opportunity to merge without disrupting through traffic, such as at signalized intersections. Acceleration lanes should also be avoided at intersections with poor geometry that results in significant skews and differences in grade that affect the drivers ability to properly navigate the acceleration lane. The use of acceleration lanes should generally be restricted to rural, free flow, or controlled access situations where the requisite acceleration lane length can be provided.

4.6 Grade Separated Intersections and Interchanges The safest, most efficient manner of accommodating traffic for all road users on intersecting roads occurs when the intersecting roads are grade separated at grade separated intersections or interchanges. The selection of the appropriate type of grade separation or interchange and its design is influenced by many factors including highway functional classification, composition of traffic, design speed and access control. In addition to these controls, economics, terrain and right of way are important in designing facilities with adequate capacity to accommodate traffic on the intersecting roads. A ramp includes all types, arrangements, and sizes of turning roadways that connect two or more intersecting roads at a grade separated intersection or interchange. Typically, the horizontal and vertical alignment of ramps is based on design speeds lower than the intersecting roadways. Additionally, ramp lengths are based on the design speed of the intersecting roadway, the merge speed of vehicles on the ramp, and the grade of the ramp as it enters the intersecting roadway. The ramp width is governed by the curvature of the ramp, and the volume and type of traffic. It is important to note that the ramp roadway width includes the traveled-way width plus the requisite shoulder width.

4.7 Median Openings Median opening designs range from designing for simple U-turn movements to more complex unsignalized and signalized rural and urban intersections that may include traffic from minor crossroads and streets or major roadways and commercial entrances. The design of median openings and median end treatments is based on traffic volumes, operating speeds, predominant types of turning vehicles and median width. Crossing and turning traffic must operate in conjunction with the through traffic on a divided highway which makes it necessary to know the volume and composition of all movements occurring simultaneously.

4.8 Traffic Control Devices Traffic control devices, such as signs, markings, and signals, are essential to establish and maintain effective traffic operations at intersections. The extent of such traffic control may range from a stop sign at a simple road approach to a complex system of synchronized traffic signals on a high-volume urban arterial. Consideration should be given to the need for traffic control devices during the geometric design of intersections, particularly those that carry considerable traffic volume with many turning movements. The needed types of traffic control devices may influence the shapes of turning roadways and traffic islands. Designers need to consider effective placement of signs and signals as well as locations of cross walks where pedestrians are present. Designers shall refer to the Delaware MUTCD for a comprehensive discussion and direction in the use of traffic control devices and their application to intersection design.

4.9 Roundabouts A roundabout is a form of circular intersection in which traffic travels counterclockwise around a central island and in which entering traffic must yield to circulating traffic. The most commonly used roundabouts are single lane roundabouts, multi-lane roundabouts and mini roundabouts. Roundabouts in all three categories should be designed with pedestrian and bicycle facilities. They should also be designed for low entering speeds for entering vehicles and provide accommodations for the appropriate design vehicle. Designers shall refer to NCHRP Report 672, Roundabouts: An Informational Guide, 2nd Edition for a comprehensive discussion on the concepts and design of roundabouts.

Chapter 5: Miscellaneous

5.1 Multi-Modal Facilities As part of improving the transportation network, DelDOT initiates and designs projects to enhance multi-modal transportation. These projects include the addition of sidewalks, shared use paths, bicycle facilities, transit facilities and park-and-ride lots. These projects enhance multi-modalism and multi-modal systems while also decreasing vehicular traffic and automobile emissions. Consistent with DelDOT’s Complete Streets Policy, all new roadway projects, reconstruction projects, and projects that widen the pavement shall consider all modes of transportation and accommodate accordingly. System maintenance projects should strive to improve pedestrian, bicycle, and transit facilities within their limits and project scope if feasible.

5.1.1 Pedestrian Facilities The transportation system should provide a safe network of facilities to accommodate pedestrians. Pedestrian facilities are a key connection point in the design of multi-modal systems. Pedestrian facilities benefit both pedestrians and motorists by creating separation between pedestrian and vehicular paths. Accordingly, pedestrian facilities are an integral part of DelDOT’s transportation infrastructure program. Pedestrian facilities provide safe and reasonable access to public transportation and other adjacent land uses and communities. Designers should be aware that pedestrian facilities located along divided highways with significant traffic and pedestrian volumes often require the installation of a median barrier to direct pedestrians to designated crossing areas.

All sidewalk facilities should be designed and planned in accordance with the latest version of the DelDOT Pedestrian Accessibility Standards for Facilities in the Right of Way and the AASHTO Guide for Planning, Design, and Operation of Pedestrian Facilities. For projects where pedestrian access is not the primary purpose, the designer should still consider making reasonable connections to logical pedestrian termini, even if that requires a modest expansion of the project limits.

5.1.2 Bicycle Facilities There is a wide range of facility improvements that can enhance bicycle transportation. In general, additional travel lane or shoulder width can increase the suitability of roadways for bicycling. Designation of bicycle lanes with appropriate signs and pavement markings will help increase the predictability of both bicycle and motor vehicle movements. Additional separation of bicycle traffic from motor vehicle traffic on shared use paths is generally desirable on most suburban roads to provide low stress bicycle facilities where bicyclists of varied skill levels may be present. It is important to note that pedestrians are permitted to use shared use paths and accordingly, shared use paths need to be designed in accordance with the latest version of the DelDOT Pedestrian Accessibility Standards for Facilities in the Public Right of Way. All bicycle facilities should be designed in accordance with the AASHTO Guide for the Development of Bicycle Facilities in conjunction with the Delaware MUTCD.

5.1.3 Bus Facilities Bus transit facilities and bus stops should be identified and where appropriate included in transportation projects. Bus stops are generally located at or near major trip generators, destinations or at regular intervals based on population density. Bus stops are located where passengers can board and depart safely and where buses can safely enter and exit the traffic flow. For specifics, designers shall refer to the latest version of Delaware Transit Corporation’s: Bus Stop and Passenger Facilities Policy.

5.2 Landscaping and Reforestation Act The Landscaping and Reforestation Act states that “forested land in the State, together with landscape features such as trees, shrubs, ground covers…not only improve the aesthetic value of our State, but carry with them valuable benefits to the health and welfare of our citizens and our environment.” The Act also states: “It is likewise declared that the Department of Transportation is a leader in replacing forestlands that are required to be cleared for such projects and in providing travelers throughout the State with scenic vistas along its roadways while maintaining safe design and construction standards.” The requirements of the Act shall be invoked whenever the Department of Transportation performs a construction project. A “construction project” is defined as any activity undertaken, authorized, or required by the Department of Transportation through which any expressway, arterial or collector road is constructed on a new alignment, or widened by adding one or more through travel lanes. Designers shall refer to the Design Resource Center for guidance and specific standards when projects require implementation of the Landscaping and Reforestation Act.

5.3 Roadway Drainage Adequate drainage is essential in the design of roadways since it affects the roadways serviceability and usable life, including the integrity of the pavement. Safety concerns such as ponding on the roadway are also addressed with adequate roadway drainage. Adequate drainage involves providing drainage systems that collect, transport, and remove runoff from the roadway. Drainage systems can be classified as either open drainage systems or closed drainage systems. Open drainage systems typically convey stormwater to an outfall in roadside or median ditches. These systems are preferable based on the minimal cost and minimal maintenance required. However, in certain situations, especially for urban highways, it may not be practical to construct an open drainage system to collect pavement runoff and convey it to an outfall point. In these cases, a closed drainage system is utilized to convey stormwater to an outfall point. Closed drainage systems typically include curbs or curb and gutter combinations along the edge of pavement for containing the runoff and channelizing the flow into inlets through which the pavement runoff is removed. The most current editions of Hydraulic Engineering Circular No. 22 – Urban Drainage Design Manual and the AASHTO Drainage Design Manual shall be used for all elements associated with designing drainage systems on roadway projects in Delaware.

5.4 Pavement Subbase Drainage Keeping the pavement subbase and underlying soil dry is a significant design consideration. Excessive moisture combined with increasing traffic will inevitably lead to premature pavement distress. Water can enter the pavement structure from many directions including poorly sealed or deteriorated joints, surface cracks, or high-water tables. If water is trapped within the pavement structure, pavement performance will be affected through loss of support due to erosion of any granular material and loss of material strength. Prevention and timely removal of water intrusion within the pavement structure are important steps to consider in extending the life of pavement sections on DelDOT projects. Designs should include proper pavement cross slopes, freely draining soils and proper ditching where possible.

5.4.1 Underdrains Underdrains are defined as a system of perforated pipes placed within the pavement section designed to collect and remove water from the pavement subbase. Underdrains are typically a wise investment when designing pavement sections as removal of water from the pavement subbase has a direct impact on extending the life of pavements. In low, poor draining areas with poorly draining soils, the use of underdrains has proven to be very effective in the removal of water from pavement subbases. On most higher volume roads designers should include underdrains in the pavement section for new construction and reconstruction projects. However, sound engineering judgment should be used when deciding to implement underdrains on projects. For example, underdrains may often be omitted on projects constructed on higher fill sections with well-draining soils. Designers shall refer to the DelDOT Standard Construction Details and the DelDOT Standard Specifications for additional guidance on installation of underdrains.