7 - Motor Vehicle Facilities Supporting Multimodal Accommodation
Published: January 20, 2023
This chapter provides an overview of how typical roadway elements can be designed to support a multimodal transportation network. Other chapters in this Guide provide guidance on addressing pedestrian and bicyclist travel and behavior, whereas this chapter focuses on addressing motorist travel and behavior. Designers should consider all existing and planned modes during the design process. As discussed in Chapter 3, this process may require design flexibility when applying roadway design criteria.
7.1.1 Design Principles and Objectives
ODOT’s mission is to provide safe and easy movement of people and goods from place to place. Based on existing data, crashes involving motor vehicles with pedestrians or bicyclists are disproportionately higher by volume than crashes only involving motor vehicles, and when crashes occur at conflict points with higher speeds the severity of injury increases. Roadway designers should address these systemic safety issues to reduce the frequency and severity of crashes where they are likely to occur.
Designers should refer to the Multimodal Planning Goals described in Chapter 2 for an understanding of the principles that guide active transportation planning, facility selection, design, and project prioritization.
7.2 Minimizing Turning Speeds at Intersection
Vehicle turning movements affect operations and safety at an intersection. If conflict points cannot be eliminated through signal phasing or grade separation, intersection design should minimize the speed differential between users at the points where travel movements intersect. The process for controlling the speed of right turning vehicles is described in Sections 7.2.1 through 7.2.6. For controlling left turning vehicle speed, see Section 7.2.7. Raised crossings may also be considered at intersections and have been shown to reduce turning vehicle speeds of both right and left turning vehicles (see Section 4.5.5).
7.2.1 Intersection Design and Check Vehicles
For roadways where the most common vehicle is a passenger car, delivery vehicle, or single unit truck, designing intersections to easily accommodate larger vehicles with large turning radii can negatively affect crossing distances, exposure to conflicts, speed of common turning vehicles, and can impact right-of-way or be inconsistent with the surrounding land use context. Similarly, using a smaller design vehicle at intersections regularly used by larger vehicles should be avoided because frequent operational challenges may occur, may lead to encroachment beyond the edge of pavement or curbline, and can lead to damage to infrastructure such as curb ramps, signs, or poles.
The L&D Manual Volume 1 Section 401.9 defines the two types of vehicles that must be considered at each intersection: Intersection Design Vehicles (IDV) and Intersection Check Vehicles (ICV). Figure 401-15a shows the recommended IDVs and ICVs along with their acceptable Degrees of Encroachment (DE) for ODOT roadways based on the functional classifications of the intersecting roadways. On many local roadways, particularly in residential areas, the design and check vehicles will be smaller than those stated in Figure 401-15a. In addition, where the intersection check vehicle is an emergency service vehicle, a greater degree of encroachment may be considered which may require sloped (mountable curbs). Designers should check local policies for selecting IDV and ICV and adjust based on the roadway type and surrounding land use context.
7.2.2 Turning Vehicle Design Speed
At both signalized and unsignalized intersections (including roundabouts), steps should be taken to ensure that turning speeds are kept low and that sight distance is not compromised for either the pedestrian, bicyclist, or motorists. While performing swept path analyses, the maximum recommended turning speed of the design and check vehicle is 10 mph.
7.2.3 Actual and Effective Curb Radius
Two distinct radii need to be considered when designing street corners. The first is the actual radius of the street corner itself, and the second is the effective turning radius of the selected IDV or ICV, see Figure 7-1. The effective turning radius is the radius needed for a turning vehicle to clear any adjacent parking lanes and/or to align itself with its new travel lane. Using an effective turning radius allows a smaller curb radius than would be required for the motorist to turn from curb lane to curb lane.
Figure 7-1: Actual Corner Radius vs. Effective Turning Radius
7.2.4 Designing Intersection and Driveway Corner Radii
Designers should strive to provide the smallest appropriate corner radius for the given IDV and ICV, target turning speeds, acceptable lane encroachment, number of receiving lanes, and effective pavement width. In addition to discouraging higher turning speeds, smaller corner radii are preferred in order to better align curb ramps with pedestrian paths of travel and shorten crossing distances.
To achieve the smallest appropriate corner radius, designers should follow these strategies:
- Using vehicle turning software or turning template, designers should minimize the corner radius while accommodating the effective turning radius of vehicles.
- Where pedestrians or bicyclists are expected and the effective turning radius exceeds 15 ft., consider the following:
- Push back the stop line of the receiving street beyond the minimum 4 ft. from crosswalks where appropriate. Ensure that any encroachment does not conflict with overlapping phases at signalized intersections. In general, stop lines should not be pushed back more than 30 ft. from crosswalks as motorist compliance may be diminished; however, the maximum distance from stop line to traffic signals cannot exceed the sight distance and clear zone requirements established in OMUTCD Chapter 4D.
- Provide a truck apron to increase the effective radius of larger vehicles, including SU-30, while providing a smaller effective radius for the majority of vehicles (e.g., passenger car). See Section 7.2.5 for additional information and design guidance.
- At skewed intersections and where truck aprons would exceed 15 ft., consider a channelized island as described in Section 7.2.6.
- Consider a raised crossing to slow turning vehicles. See Section 7.8.3.
As described elsewhere in these guidelines, curb extensions are beneficial to pedestrians. It is acceptable to have a curb bulb with a larger curb radius that shortens crossing distances while accommodating large vehicles. For curbless streets, care should be taken at corners to ensure that proper design treatments are included to identify safer turning distances for large vehicles. Such treatments may include pavement coloring, different materials, and other features that provide a visual indication of the apex of the turn.
Flex delineator posts or engineered rubber curbs may be used as an interim treatment to reduce larger corner radii. When used, they should be placed at least 1 ft. offset from the turning radius of design vehicles at all intersections and driveways.
7.2.5 Truck Aprons
Truck aprons are most common within the center island of a roundabout but should also be considered at intersection corners to accommodate the turning characteristics of larger vehicles while slowing the turning speeds of smaller design vehicles. The truck apron must be designed to be mountable by larger vehicles to accommodate their larger turning radius while smaller vehicles follow the smaller radius along the outside edge of the truck apron, see Figure 7-2.
The outside edge of a truck apron (i.e., closest to the travel lane) is constructed using a mountable curb and should be designed so that passenger vehicles follow this mountable curbline at the desired 10 mph speed. Larger vehicles, including SU-30, can traverse the truck apron if desired, but the control vehicle should be used to specify the effective radius.
The truck apron is part of the motorist travel way. Truck aprons shall not extend through bike lanes or crosswalks unless they are designed to accommodate these users, and bicycle stop bars and pedestrian accommodations (e.g., curb ramps, crosswalks) must be placed to prevent these users from waiting in the travel way. Colored concrete and/or pavement markings should be used within the truck apron area to provide a visual contrast from the adjacent roadway and sidewalk. This communicates to drivers of smaller vehicles that this is not an area to drive over. Where widths exceed 15 ft., the intended use of the apron may not be clear and designers may consider a channelizing island to limit the street crossing distance for pedestrians and bicyclists (see Section 7.2.6).
In retrofit conditions, a truck apron that extends all the way to the existing curbline may not be possible without significant stormwater system modifications. In these situations, truck pillows that maintain drainage along the existing curbline may be more practical and feasible.
See DWG 7-1 for additional design details for truck aprons and truck pillows.
An edge line should be provided along the outside edge of wider truck aprons to ensure that the path of travel is visible. Gore markings may be installed on the truck apron itself, but this is often unnecessary if colored pavement is used, see Figure 7-3.
Where buses are likely to traverse the truck apron with regularity (such as transit routes), truck aprons should be designed to allow the bus to complete the turn without traversing the truck apron. A tiered truck apron with a curb reveal from 0 to 1 inch can be constructed for use by buses while the second tier can be designed with a 3 inch curb reveal for use by larger trucks, see Figure 7-3.
Figure 7-2: Typical Truck Apron Layout at a Protected Intersection
Figure 7-3: Truck Apron with Concrete and Pavement Markings (top) and Tiered Truck Apron with Colored Concrete (bottom)
7.2.6 Turning Lanes and Channelized Islands
A channelized right turn lane may be preferable to a larger corner radius to decrease the street crossing distance for pedestrians and bicyclists. These are common at skewed intersections or other roadways with large curb radii and cross-sections.
Where channelized right turn lanes are provided, it will be necessary to choose between a flat angle (> 140 degrees) or a right-angle (< 120 degrees) approach to the cross street (see Figure 7-4). To ensure the channelized right turn lane promotes motorists yielding at pedestrian and bicycle crossings, as well as to cross street motorists, a right-angle entry design is preferred. This design encourages lower turning speeds and improves the driver’s view of people approaching crosswalks and motorists approaching from the cross street.
Figure 7-4: Channelized Right Turn Design Options
The use of flat-angle approaches should be avoided, except where:
- motorists have their own receiving lane on the cross street, and
- higher turning speeds are necessary for safe merging onto the cross street, and
- it is needed to accommodate the turning path of the design vehicle (e.g., at skewed intersections), and
- pedestrian and bicycle crossings are not allowed or infrequent, or traffic controls devices are provided to control the conflicting motor- vehicle movement and facilitate pedestrian and bicyclist crossings.
A raised crosswalk (see Section 4.5.5) should be considered at channelized right turn lanes where motorists do not face stop or traffic signal control to encourage motorist yielding. They may also be beneficial at yield, stop, and signal control intersections where it is desirable to reduce encroachments into the crosswalk. When used at a channelized island, the crosswalk should be located to allow one vehicle to wait between the crosswalk and the cross street.
L&D Manual Volume 1, Section 401.7.2, provides additional guidance on the design of channelized islands.
7.2.7 Median Islands and Hardened Centerlines
Chapter 4 discusses how a raised median island can be used to provide pedestrian refuge space to cross a major street. In that situation, a minimum of 6 ft. is required to accommodate a pedestrian or bicyclist waiting to cross the second portion of the crossing. When less than 6 ft. in width is available, designers can still provide a center median, also known as a hardened centerline, to channelize and slow the speeds of left turning motorists as they prepare to cross the path of pedestrians and bicyclists.
A hardened centerline is comprised of a painted centerline supplemented by flexible delineators, mountable curb, rubber curb, concrete curb, IN-STREET PEDESTRIAN CROSSING signs (R1-6), or a combination of these treatments. The dimensions of a hardened centerline will depend on the intersection geometry and vehicle turning radius. Hardened centerlines should be considered where higher-speed left turns occur concurrent with pedestrian and/or bicyclist movements, as they have been found to reduce the speed of left turning motorists by reducing the effective turning radius.1 Hardened centerlines can be appropriate on both the departure roadway and the receiving roadway to control the left turning motorist path of travel. See Figure 7-6 and Figure 7-7.
Figure 7-6: Example of Hardened Centerline Applications with Flexible Delineators on the Departure Roadway and a Pedestrian Crossing Island on the Receiving Roadway
Figure 7-7: Flexible Delineators and Hardened Centerline to Control Turning Speed
7.3 Vehicle Travel Lane and Shoulder Widths
L&D Manual Volume 1, Section 301.1.2, Fig. 301-2 and Fig. 301-4, provide the lane widths for rural and urban areas, and Section 301.2.3, Fig. 301-3 and Fig. 301-4, provide guidance on shoulder widths for rural and urban areas.
When pedestrians or bicyclists are expected to be present, designers should use the minimum widths as described in the L&D Vol. 1 for travel lanes, on-street parking, and potentially shoulders. Deviations from the minimum lane and shoulder widths may be considered where appropriate to achieve the project goal(s). Narrow lane widths discourage higher operating speeds, reduce pedestrian crossing distances, and create spaces for sidewalks and bikeways. FHWA no longer considers lane and shoulder width as controlling criteria for roadways on the National Highway System (NHS) with less than 50 mph design speeds, allowing for increased flexibility to implement narrower lanes on those roadways. For roads on the National Network (or National Truck Network) a design exception for lane width is required if at least one 12 ft. lane in each direction cannot be provided. See Section 105.3 of the L&D Vol. 1 for more information. In retrofit situations where excess pavement cannot be reduced, travels lanes can be visually narrowed with pavement markings or contrasting materials. This space could be used for a buffer for bike lanes, a marked center median, or to provide space for a designated shoulder or bike lane.
In constrained settings, sidewalk or bikeway buffer zones could accommodate traditional shoulder functions of drainage, snow storage, and lateral support of pavement.
These concepts are further discussed in Section 7.5.2.
7.4 On-Street Parking Considerations
Section 6.3 provides details on the design and considerations of on-street parking in relationship to bicycle lanes. Guidance on parallel and angled parking is provided, as well as an overview of considerations for loading zones and accessible parking spaces. Where angled parking can be accommodated, back-in angled parking is preferred as it improves driver sight lines when exiting parking spaces. Additionally, back-in angled parking directs passengers exiting the vehicle back towards the curbline (i.e., the opened vehicle doors block access towards the street) which can beneficial to direct children exiting vehicles towards the sidewalk.
Sections 4.6 and 6.3.7 provide guidance for the design of accessible on-street parking spaces. Section 7.7 provides additional guidance on restricting parking near intersections, driveways, and crosswalks.
7.5 Reallocating Street Space
Streets and highways can be retrofitted to better accommodate bicyclists and other users by widening the roadway or by reallocating pavement to these users by reconfiguring the travel lanes. This approach can better balance the level of service for all travel modes by providing wider sidewalks, bike lanes, and buffers between bicyclists, pedestrians, and motor vehicles. Space can also be dedicated to plantings and amenity zones and reducing crossing distances.
When retrofitting roads for bicycle facilities, the width guidelines for bike lanes and paved shoulders (see Sections 6.3) should be applied. For additional information on retrofitting bicycle facilities on existing streets and highways see the FHWA Incorporating On-Road Bicycle Networks into Resurfacing Projects and the FHWA Road Diet Information Guide.
Retrofitting bicycle facilities on bridges presents special challenges because it may be impractical to widen an existing bridge. The guidance below for retrofitting bicycle facilities without roadway widening is applicable to existing bridges. Further guidance on accommodating bicyclists on bridges is presented in Chapters 5 and 6.
7.5.1 Retrofitting Bicycle Facilities by Widening the Roadway
Where right-of-way is adequate, or where additional right-of-way can be obtained, roads can be widened to provide the desired multimodal facilities. The design should follow the design principles discussed throughout this guide and ensure that widening the roadway does not result in increased vehicle speeds that may adversely impact bicyclists and pedestrians.
7.5.2 Retrofitting Bicycle Facilities Without Roadway Widening
In many areas, especially built-out urban and suburban areas, physical widening is impractical, and bicycle facility retrofits are constructed within the existing paved width. There are three methods of modifying the allocation of roadway space to improve bicyclist accommodation:
- Reduce or reallocate the width used by general purpose travel lanes.
- Reduce the number of general purpose travel lanes.
- Reconfigure or reduce on-street parking, including parking lane width.
Travel lane widths can often be narrowed without any significant changes in levels of service for motorists. Before a reduction or reallocation in the number of travel lanes or their widths is considered, an operational study must be performed to evaluate the impact of the proposed changes on the level of service. One benefit is that bicycle LOS will be improved when adding a bicycle facility. Creating shoulders or bike lanes on roadways can improve pedestrian conditions as well by providing a buffer between the sidewalk and the roadway.
Narrowing existing motor vehicle lanes may create enough space for separated bike lanes, widening sidewalks and buffers, or a combination of on-street bike lanes and enhancements to the pedestrian realm. Lane widths that are greater than the minimum values shown in L&D Vol. 1, Figures 301-2 and 301-4 and, depending on condition, may be candidates for narrowing. Narrower lanes can contribute to lower operating speeds along the roadway, which may be appropriate in dense, walkable corridors or near transit in more rural areas.
Figure 7-8: Lane Narrowing Scenarios
Lane reductions, or road diets, are most typically done on roadways with excess capacity where anticipated traffic volumes have not materialized to support the need for additional travel lanes.
The most common road diet configuration involves converting a four-lane undivided road to three lanes: two travel lanes with a turn lane in the center of the roadway, also known as a Two-Way Left Turn Lane (TWLTL)—see L&D Manual Volume 1, Section 402 for more information. The center turn lane often improves safety and reduces motor vehicle delay by giving turning vehicles that previously blocked the through lanes their own turning lane.
The space gained for a center turn lane is often supplemented with painted, textured, or raised center islands. If considered during reconstruction, raised median islands may be incorporated in between intersections (See Section 4.5.3).
Road diets are most appropriate on roadways with the following configurations:
- Four-lane streets with volumes less than 15,000 vehicles/day are generally good candidates for four- to three-lane conversions.
- Four-lane streets with volumes between 15,000-25,000 vehicles/day may be good candidates for four- to three-lane conversions. Refer to FHWA Road Diet Informational Guide.
- Six-lane streets with volumes less than 35,000 vehicles/day may be good candidates for six- to five-lane (including a center two-way left turn lane) conversions.
A traffic study must be conducted to evaluate potential reductions in crash frequency and severity, to evaluate motor vehicle capacity and level of service, to evaluate bicycle LOS, evaluating driveway density and impacts to LOS, and to identify appropriate signalization modifications and lane assignment at intersections before implementing a road diet.
7.6 Driveways and Alleys
See Section 4.5.5 and 4.5.7 for discussions of pedestrian accessibility and motorist speed management at driveways and alleys.
7.7 Crosswalks at Uncontrolled Intersections and Mid-block Locations
See Sections 4.4, 5.6, and 6.5 for discussions of pedestrian and bicyclist design considerations at uncontrolled and mid-block locations.
7.8 Speed and Volume Management for Multimodal Design
People walking and bicycling are particularly vulnerable in the event of a crash, and vehicle speeds where conflicts occur are a primary factor in the likelihood of serious injuries and fatalities (see Figure 7-9). Where vulnerable road users are present and in urban contexts, ODOT’s speed zone calculations will be derived from the 50th percentile speed as determined from a speed zone study. For information on conducting a speed zone study, refer to Section 1203 of the TEM.
Figure 7-9: Vehicle Speeds and Risks to Pedestrians2
Design speeds are discussed in L&D Manual Volume 1, Section 104. Where pedestrian and bicycle facilities are provided on low-speed facilities, design speeds should be equal to the legal speed. Using a design speed higher than the legal speed may cause motorists to feel comfortable exceeding the posted speed. When motor vehicle operating speeds create a safety concern for people walking or bicycling, designers should reduce the number of conflict points and overall exposure of pedestrians or bicyclists. Where conflicts remain, designers should increase visibility (e.g., sight distances and illumination) and provide ample warning to both motor vehicles and more vulnerable users. On existing roadways with operating speeds that exceed the legal speed, roadway redesign and traffic calming measures should be considered to reduce speeds and improve safety and comfort for all users.
Traffic calming devices help to manage motor vehicle operating speeds and volumes by introducing changes to the roadway cross-section (horizontal deflection), pavement elevation (vertical deflection), reductions in roadway width, alterations to typical motor vehicle travel patterns, or a combination of these techniques. By managing speeds, reinforcing target design speeds, and managing vehicle volumes, traffic calming increases the comfort of a shared operating environment and minimizes conflicts between users. Traffic calming devices are not appropriate in every context. The following sections discuss both the application and design of traffic calming devices.
The addition of traffic calming treatments can result in through traffic trips diverting to other nearby streets. While this may be preferable for bicycle boulevards (see Section 6.3.2), it may create undesirable conditions on other parallel streets. To avoid unintended behaviors, traffic calming should be considered from a roadway network perspective.
The use of traffic calming devices should consider the following:
- Design speed should target the posted speed.
- Designs shall consider both the design and check vehicle.
- Some vertical deflection treatments may have an adverse effect on emergency response vehicles. Many types of vertical deflection can be designed to accommodate transit and emergency vehicles by leaving gaps that match the wheelbase.
Traffic calming treatments are most effective when they are deployed at regular intervals ranging from 200 to 400 ft. between treatments.
7.8.2 Horizontal Deflection
A horizontal deflection hinders the ability of a motorist to drive in a straight line by creating a horizontal shift in the roadway. This shift forces a motorist to slow the vehicle in order to comfortably navigate the measure. Horizontal deflection is most appropriate on local and collector streets. Lateral shifts and mini roundabouts may also be appropriate on thoroughfares. See L&D Manual Volume 1, Section 202 and Section 301.1.4, for allowable horizontal deflection and curve design criteria and transition/taper rates.
Lateral Shifts & Chicanes
Lateral shifts cause travel lanes to shift in one direction, often by shifting on-street parking from one side of a street to the other side of the street. Chicanes are a series of curb extensions, pinch points, parking bays, or landscaping features that alternate from one side of the road to the other to establish a serpentine path of travel for motorists along a street. The following design guidance should be considered for both treatments:
- Lateral shifts and chicanes can be used on two-way streets with one lane in each direction, and one-way streets with no more than two lanes.
- Traffic-calming effects are greatest when deflection shifts vehicles back and forth by at least one full lane width.
- The shifting taper of horizontal deflections should be based on the posted speed. Provide advisory speed plaques (W13-1P) where appropriate to supplement Horizontal Alignment Signs (See OMUTCD, Section 2C.07).
Figure 7-10: Examples of Lateral Shift (left) and Chicane (right)
- If vegetation or other features are used within chicanes, it should generally be low maintenance and not grow taller than 2.5’ to avoid impeding sight distance. In locations with mid-block pedestrian crossings, sight distances must be maintained.
- Interim chicanes can be created using a combination of pavement markings, parked cars, flexible delineator posts, or other vertical elements.
- Avoid using these horizontal deflection treatments along streets with bus, freight, or emergency response activity unless traffic volumes are very low and large vehicles can use the full roadway width.
- Bikeways should be maintained throughout to avoid abruptly squeezing bicyclists into motor- vehicle traffic on streets with higher traffic volumes, particularly in locations with uphill grades where bicyclists may be traveling slower than other roadway traffic.
Neighborhood traffic circles are primarily used at four-leg, two-lane local streets and are installed to reduce crash severity and slow traffic speeds. Splitter islands are not required on approaches (unlike a modern roundabout), and the central island is typically raised with a mountable apron to prevent a straight-through movement of the typical design vehicle. The occasional control design vehicle should not be precluded from operating within the intersection with encroachment, if necessary, which may include going the “wrong way” to the left of the traffic circle to make a left turn. Landscaping may be planted with the center median if it does not need to be traversable. The local streets typically do not have marked centerlines.
Figure 7-11: Schematic Examples of Mini-Roundabouts and Neighborhood Traffic Circle
Designers should consider the following:
- The aesthetic value of a traffic circle is an important part of its design. Well-designed traffic circles fit naturally into the neighborhood and can include landscaping, green street elements, or decorative pavement such as stamped concrete, pavers, etc.
- Traffic circles should be visible to street users with pavement marking, signing and reflectors used where appropriate. Regulatory and/or warning signage should be provided to advise traffic to proceed counterclockwise around the circle.
- Careful attention should be paid to the available lane widths and turning radius used with traffic circles to accommodate the design vehicles.
- Maintaining access to underground utilities must be considered.
Mini-Roundabouts and Modern Roundabouts
Mini-roundabouts are primarily used on two-lane collector streets and require all vehicles to yield to traffic in the roundabout and traverse counterclockwise around the circle. Because of their traffic calming effects, they are installed primarily to reduce crashes, but can also reduce delays at minor approaches. Mini-roundabouts share the characteristics of modern roundabouts (See Chapter 9 and L&D Manual Volume 1, Section 403) with approach splitter islands and yield control on each approach. The splitter island may be raised or painted. The central island may be raised with a mountable apron (See Section 7.2.5 and D Manual Volume 1, Section 403.6.4) to constrain the circulating roadway width while still accommodating larger vehicles. Where the diameter of the center circle is less than 8 ft., it may be painted. Mini-roundabouts are typically constructed to fit within existing curb lines at intersections but in some locations may require reconstruction of corner radii to accommodate larger design vehicles. In most cases the addition of new accessible curb ramps will be necessary to accommodate pedestrians at a crossing setback from the circulating roadway. The intersecting streets will typically include marked centerlines.
7.8.3 Vertical Deflection
Vertical deflections, which include speed tables, speed humps, speed cushions, and raised crosswalks, are effective means for controlling the speeds of motor vehicles. OMUTCD Sections 2C- 29, 3B.25 and 3B.26 provides guidance on markings and signage for vertical deflection treatments. Vertical deflection as a traffic calming measure is only permitted across local and collector streets where posted speeds are less than 35 mph and where roadway grades do not exceed 8 percent.
In general, all vertical traffic calming devices within roadways should be built with a bicycle friendly vertical deflection profile. The preferred profile is sinusoidal, which is easier for bicyclists to traverse than a circular or flat profile. The use of parabolic profiles should be avoided on streets with bicycle traffic, as their abrupt pavement joint edges can cause discomfort or a loss of control of the bicycle. The front edge or lip of the device should be as smooth as practical and meet the road with minimal vertical. Sinusoidal profiles are also easier for maintenance vehicles to traverse for street sweeping or snow plowing activities, and they have less of an effect on emergency vehicle access. See Table 7-1for additional guidance on vertical deflection considerations.
Table 7-1: Vertical Deflection Characteristics and Desired Motorist Speed at Crossing 3
Maximum Table Height
|Appropriate Locations||Appropriate Ramp Profiles||
Min Ramp Length and Target Slope
<= 20 MPH
6 Feet or 1:12
<= 25 MPH
|All Streets without Designated Emergency Response, Truck or Frequent Transit Routes||Sinusoidal or Straight||
6 Feet or 1:18
<= 30 MPH
|3.0 inches||Arterial or Collector Streets without Designated Emergency Response, Truck or Frequent Transit Routes||Sinusoidal or Straight||6 Feet or 1:24|
|<= 35 MPH||3.0 inches||Arterial Streets with Designated Emergency Response, Truck or Frequent Transit Routes||Straight||9 Feet or 1:24|
Where speed humps are used to control speeds along a roadway, they are most effective when they are placed periodically along the route (every 200 – 400 ft.) to reinforce speed control. These devices should be designed to maintain existing drainage patterns to avoid requiring additional inlets and storm sewer. DWG 7.2 provides additional speed hump design details, which includes tapering the speed hump near the edge of pavement or curbline to minimize retrofit installation costs and allow stormwater to flow into existing gutters. The details also show options for speed humps with gaps between separate humps. These may be desirable on bicycle boulevards, or to better accommodate emergency services vehicles, as they allow the bicyclist or the wider emergency services vehicles to straddle the hump while narrower passenger vehicles cannot avoid them. Designers should consider the existing site conditions and make adjustments where necessary.
Raised Crossings and Speed Tables
See Section 4.5.5. for speed considerations.
For both speed tables and raised crosswalk, a 10 foot minimum flat area should be provided. Speed tables are most effective when they are placed periodically along the route (every 200 – 400 ft.) to reinforce speed control.
Raised crossings require additional design considerations (see Section 4.5.5). Raised crossings may also be placed at intersections to reduce both left and right turning vehicular speeds.
7.8.4 Street Width Reduction
Research has shown that motorist speeds can be reduced by creating a sense of enclosure or by creating shared travel spaces that eliminate the perception of motorist priority.3 This can be accomplished through the provision of continuous or intermittent elements that reduce the effective travel lane width and narrow the field of view, resulting in a naturally slow-speed environment. Designers should consider additional streetscape elements to further enforce to drivers the need to drive slowly and with caution.
A road diet is the conversion of an undivided roadway to a cross-section with fewer or narrower through motor vehicle travel lanes. The elimination or width reduction of travel lanes can reduce the severity of crashing through vehicle speed reduction. It is appropriate for arterial, collector, and local streets in an urban, suburban, or rural setting. The reduction on traveled way widths may also provide space for other improvements such bicycle lanes and crossing islands. See Section 7.5.2 for more design information.
Narrow (Yield) Streets
In many communities, particularly residential neighborhoods, streets are quite narrow. Some of these streets are called “yield streets,” “queuing streets,” or “woonerfs” because they require motor vehicles to pull to the side, usually into a parking lane or driveway, to allow motor vehicles approaching in the opposite direction to pass. Yield streets are appropriate in residential environments where drivers are expected to travel at low speeds and volumes are low, see Table 7-2.
Table 7-2: Yield Roadway Motorized Traffic Volume and Speed Characteristics
Average Daily Traffic Volume (ADT)
Operating Speed (mph)
Yield streets are typically 24–28 ft. wide with parking on both sides, or 21 ft. wide with parking on one side to limit the effective width of the operating space to require motorists to pull into empty parking spaces or driveway openings to allow approaching motorists to pass. Width reduction measures may include on-street parking (See Section 7.2.7), curb extensions (See Section 7.7), median islands (See Section 4.5.3), or a combination of these measures.
For a yield street to function properly, parking density must be relatively high, but motorists must also have regular opportunities to pull to one side. In general, yield streets are most effective where on- street parking utilization falls within 40 to 60 percent of available spaces or when 40 to 60 percent of the curb space is available due to parking restriction and parking demand. When more than 60 percent of the curb is space is regularly available, curb extension or pinch points can be installed to reduce the curb space and create a similar effect. If parking demand is expected to increase, painted curb extensions may be used as an interim measure and removed as parking demand increases.
Yield streets should provide enough room for emergency vehicle access, the occasional moving van or large delivery truck, as well as school buses, trash trucks, and snow plows (when these vehicles are expected), to navigate safely.
Pinch-points are curb extensions that are constructed to narrow a two-way roadway to one travel lane which requires approaching motorists to yield to each other. This treatment should be reserved for mid-block locations along low-volume local streets. This treatment may be most appropriate on streets with more than 18 ft. of clear operating width which are functioning as two-lane streets, streets with low parking demand, or streets with parking on one side.
Designers should consider the following:
- To function effectively, the width of the opening should not allow two cars to pass: 14 ft. is effective and allows emergency services to navigate the opening.
- The one-lane portion may align with one direction of travel to prioritize that direction or may be centered in the roadway with no directional priority.
- A warning sign (ROAD NARROWS) and YIELD signs (R1-2) may be considered upstream of the pinch point.
- Any vegetation provided in the medians should be low growing and low maintenance.
- Vertical deflection (see Section 7.8.3) may be located within the single-lane section to further reduce speeds. In these instances, a bicycle bypass can be considered to maintain bicyclist speeds (see Figure 7-12).
Figure 7-12: One-Lane Pinch Point Centered in the Roadway with Speed Hump and Bicycle Bypass Lanes
7.8.5 Routing Restriction/Traffic Diversion
Traffic diversion strategies are used to reroute traffic from one roadway onto other adjacent streets by installing design treatments that restrict motorized traffic from passing through. These type strategies require a traffic study before implementation. These are often used on Bicycle Boulevards (See Section 6.3.2 and Section 6.5.6) to reduce motorist volumes but can also be used on other roadways where volumes are above desired thresholds for other bikeway types (See Section 7.2) or the expected pedestrian volumes.
The NACTO Urban Street Design and Bikeway Design guides provide additional types of volume management strategies beyond those discussed here.
Signing can be used to prohibit vehicles from entering a roadway using movement prohibition signs (R3-1, R3-2, R3-3, R3-5, R3-27, etc., or DO NOT ENTER signs (R5-1). These prohibitions can be for all hours or could be for peak hours only to optimize flow during times of heavy traffic. Signs should be supplemented with an EXCEPT BICYCLES plaque when bicyclists are permitted to perform the movements that are prohibited for motorists.
Signs may be supplemented by right and/or left turn pavement marking arrows to emphasize the restriction, but pavement markings should not be used when restrictions vary by time of day. Signs and pavement markings alone may not be effective at discouraging motor vehicle access. The following sections depict physical treatments that can be used to supplement signing and markings.
A diverter is an island built at an intersection to alter the movement of through and/or turning vehicle traffic. Diverters are commonly designed to maintain through travel for people walking and bicycling while altering routes for motor vehicles. For all diverters, designers should consider the following:
- Diverter islands are designed to maintain bike and pedestrian access by providing cut-throughs. Standard cut-through width for bicyclists is 6 ft.
- Diverter islands can include a combination of public art or other vertical elements, so long as they keep sight lines clear for bicyclists. Other vertical elements such as signing, flexible delineator posts, etc. may be appropriate to make the features more visible to motorists and assist snowplow operators when clearing roadways.
- A diverter’s effectiveness at limiting speeds is generally limited to the intersection where it is installed. The street may require additional traffic-calming treatments in addition to the intersection treatments to achieve the desired operating characteristics.
- Diverters must be designed with transit and emergency vehicle navigation in mind. In some cases, emergency vehicles must be able to travel over or through the diverter if gaps are spaced to accommodate them or if breakaway gates are used.
Diagonal diverters (Figure 7-13) are the most common form of full diversion. Diagonal diverters connect diagonal corners creating two disconnected streets. Vehicles are forced to turn, while through travel is maintained for bikes and pedestrians.
Figure 7-13: Diagonal Diverter
Median island diverters (Figure 7-14) are a common form of diversion used when minor roadways that include bikeways intersect with major roadways. Median island diverters restrict motorist left turns from the major roadway and restrict the motorist through movement along the minor roadway. They can also provide a crossing island for bicyclists so that the major street crossing can be accomplished as two separate crossings.
Figure 7-14: Median Island Diverter
Forced Turn Diverters
A forced turn diverter, or directional closure (Figure 7-15), is a curb extension or vertical barrier extending to approximately the centerline of a roadway, effectively obstructing (prohibiting) one direction of traffic.
Bicycles are typically permitted to travel through a directional closure in both directions, including the direction in which motor vehicle traffic is obstructed. In some cases, gaps or a contra flow bicycle lane are used to provide bicycle access.
Figure 7-15: Forced Turn - Directional Closure
A second type of forced turn diverter is a right-in/right-out island (Figure 7-16), which prohibits the through and left turn movements. This treatment can be achieved with less infrastructure than the directional closure described above and can better accommodate two-way motor vehicle traffic but may result in higher motor vehicle volumes.
Figure 7-16: Forced Turn - Right-In/Right-Out
Chapter 7 Endnotes
- New York City Department of Transportation. Don’t Cut Corners: Left Turn Pedestrian and Bicyclist Crash Study. New York City, August 2016.
- Tefft, B. C. Impact speed and a pedestrian’s risk of severe injury or death. Accident Analysis & Prevention. 50. 2013.
- City of Los Angeles, Bureau of Engineering, Department of Engineering. City of Los Angeles Supplemental Street Design Guide, May 2020.