Fast vehicle speeds reduce the comfort and safety of people walking, biking and using other mobility devices, particularly when there is minimal lateral or physical separation from vehicles. When motorists drive at faster speeds their cone of vision is narrowed and the distance they need to stop increases. This raises the risk of collision, particularly where there are higher numbers of people using the street. If a collision does occur, the probability of a serious injury or fatality rises as vehicle speeds increase. So, a critical component of creating a safe and comfortable active transportation network that attracts people of all ages and abilities is managing vehicle speeds where these users are anticipated. There are many strategies agencies can use to reduce vehicle speeds. An effective approach to setting suitable speed limits for multimodal roadways is establishing target speeds.
Target speed is the speed at which people are expected to drive and is determined for each street based on context and the street’s role within the transportation network. Per the Institute of Traffic Engineers (ITE; Designing Walkable Urban Thoroughfares: A Context Sensitive Approach, 2010), the target speed should be set at “the highest speed at which vehicles should operate on a thoroughfare in a specific context, consistent with the level of multimodal activity generated by adjacent land uses to provide both mobility for motor vehicles and a safe environment for pedestrians and bicyclists.”
Target speeds, and by extension posted speed limits and design speeds, should balance the needs of all anticipated street users. This is based on the context that lower speeds are used in areas where higher rates of people walking and bicycling are desirable.
Design speed is used to determine the design of geometric features of the roadway, which ultimately determines the speed at which people drive. Design speed should generally be selected so that the resulting prevailing speed matches the target speed. The 2018 AASHTO Green Book states “lower speeds are desirable for thoroughfares in walkable, mixed-use urban areas and this desire for lower speeds should influence the selection of the design speed. For design of such streets, a target speed should be selected. The target speed is the highest speed at which vehicles should operate on a thoroughfare in a specific context, consistent with the level of multimodal activity generated by adjacent land uses, to provide both mobility for motor vehicles and a desirable environment for pedestrians, bicyclists, and public transit users. The target speed is intended to be used as the posted speed limit.”
It also recognizes that design speeds used to establish geometric design criteria may have limited influence in urban areas. During periods of peak traffic, the operating speeds of vehicles is “regulated more by the presence of large volumes of vehicles and by traffic control devices, rather than by the physical characteristics of the street.” During non-peak traffic, operating “speeds on arterial streets are governed by such factors as posted speed limits, midblock turns into and out of driveways, intersectional turns, traffic signal spacing, and signal timing for progression.”
Therefore, when considering the selection of a design speed (and corresponding target speed), “when arterial street improvements are being planned, factors such as future posted speed limits, physical and economic constraints, and running speeds likely to be attained during off-peak hours should be considered.”
It is recommended the target speed be set to 30 mph or lower to ensure the safety of people walking and bicycling where that activity is desired and frequent interaction with motorists on the roadway is likely. As is feasible, measures (examples of which are listed below) should be considered to reduce the design speed to match the target speed.
Speed limits should match the target speed, but are most often determined by the prevailing speed (i.e., 85th percentile speed or current operating speed). FHWA’s Methods and Practices for Setting Speed Limits: An Informational Report describes methods for setting speed limits. This report acknowledges that “The original research between speed and safety which purported that the safest travel speed is the 85th percentile speed is dated research and may not be valid under scrutiny.” Much of the research on speed limits relationship to behavior were based on high-speed roadways and is thus not applicable to urban contexts.
Alternatives to the 85th percentile method include USLIMITS2 and the Safe Systems approach (which targets a safe operating speed for all users). The USLIMITS2 approach suggests the use of the 50th percentile speed in locations where pedestrian and bicycling activity is desired. It also takes into consideration crash history.
Lowering speed limits without making changes to the roadway or traffic control (i.e., lowering the design speed) may be ineffective at slowing traffic. ITE outlines 10 measures that can be used to lower design speeds to help achieve appropriate target speeds:
Other speed reduction strategies to consider depending on roadway and land use characteristics include:
Speed reduction strategies should be thoughtfully implemented to address the context and observed behavior and to consider and address potential spillover effects (i.e. traffic diverting to nearby streets). Such strategies are best approached at a neighborhood scale to proactively address spillover effects In some cases, prior to permanently implementing a traffic calming measure. It may be useful to introduce a temporary measure using paint, cones, and other low-cost material to evaluate effectiveness.
Speed limit sign.
Reducing the speed of motor vehicles turning right or left at an intersection helps to reduce the potential for conflicts with people crossing the street, as well as the severity of collisions if they do occur. As discussed under Intersections and Street Crossings, reducing the curb radius at intersections and raised crossings are two strategies for reducing the speed of turning vehicles, particularly vehicles turning right. Two strategies that have proven effective at slowing the speed of left-turning vehicles are “hardened centerlines” and “slow turn wedges.” New York City pioneered the use of these treatments after collision data revealed that collisions between pedestrians and left-turning motor vehicles were among the most common and most severe pedestrian collisions. This is common in many cities. Hardened centerlines consist of a curb and delineators placed on the centerline of the receiving leg of the intersection, forcing drivers to slow down to execute a sharper left turn rather than “cut the corner.” Slow turn wedges are installed where a one-way street meets a one- or two-way street and can consist of paint and flexible delineators or more permanent materials. Similar to the hardened centerline, slow turn wedges force drivers to execute a sharper turn at a lower speed. The two treatments can be used together for maximum effectiveness.
Gateways signal to motorists entering a commercial district, neighborhood, or main street to slow down and be more attentive to a more active street environment and the presence of pedestrians and other vulnerable roadway users. Gateway treatments encourage roadway users to interact more with the surrounding uses versus speeding through. They generally contribute to more vibrant areas that are supportive of walking and biking. Gateways can be located at major intersections or mid-block. Gateway treatments are typically implemented as part of a broader “place making” effort.
Gateway treatments often include a combination of elements. These could include wayfinding signs, enclosure elements (e.g. trees, structural features), raised crossings and intersections, traffic circles or medians, pavement treatments, lighting, plantings, and public art.
Gateway feature (Phoenix, AZ).
Lane reductions (i.e. road diets) optimize street space to benefit a broader spectrum of users when there is excess lane capacity. Lane reductions help improve safety and comfort for pedestrians, bicyclists, and other vulnerable roadway users. Reducing the number of motor vehicle lanes on a multilane roadway can help accomplish the following:
The space gained for a center turn lane is often supplemented with painted, textured, or raised center islands. If considered during reconstruction, raised center islands may be placed in between intersections to provide improved pedestrian crossings, incorporate landscape elements, reduce travel speeds, and prevent non-turning motorists from driving in the lane.
Before (l) and after (r) photos of a vehicle lane reduction (Seattle, WA).
Allowing for quick emergency response is a vital function of the roadway system. According to the National Fire Protection Association in 2016, on average only 4% of emergency calls were associated with fires while 65% were medical calls. These included traffic collisions, gun shots, cardiac arrest, etc. Street design should balance the important public safety objectives for efficient emergency response with the safety of roadway users. Emergency vehicles should be accommodated, but should not dictate the design of roadways, including corner radii, the use of traffic calming devices, and vehicle lane width. While the International Fire Code (IFC) states that roadways should have an unobstructed width not less than 20 feet, it also grants authority to the local officials to permit modifications where necessary to meet the public safety objectives of the jurisdiction which includes the design of streets to reduce vehicle collisions. The purpose of this 20-foot provision is to provide sufficient space for large ladder truck outrigger support while also providing space for fire fighters to navigate around the deployed outriggers to access the storage doors on the trucks.
Smaller fire trucks support safer street design and are more cost efficient.
Modifications to the code may include redefining what “unobstructed” means, eliminating the clear width provision from the local code, or limiting the application of the clear width to certain streets. The local agency can determine what the limits for the unobstructed width are. It may be continuous, intermittent, or it can include space beyond the curb (e.g., medians, landscaped areas) for outrigger deployment. Bike lanes may also be used for outrigger deployment. Fire departments may want to evaluate the size and function of their vehicles to help the complete streets and street design polices of the community. Many communities are choosing to purchase smaller vehicles to limit the need for wider streets and larger intersection radii to accommodate the vehicles. With very few emergency calls being directed to support large fires, the use of smaller vehicles can more effectively support the calls that are prevalent. Choosing the right sized vehicles can reduce operating costs while increasing compatibility between quick emergency response and safer street design.
Driveways create conflict points for walkers and bikers. The number of driveways and the volume of vehicles using driveways impact the potential for conflict and are important factors to consider when designing for active transportation users. Commercial corridors may have many driveways, but often these are also corridors where providing high quality access for people walking and biking is important. While there are design treatments intended to raise awareness of and reduce potential conflicts at driveways, the most effective strategy is to reduce conflicts through access management. Access management may consist of driveway consolidation and the addition of directional or full medians to better manage turning conflicts into and out of driveway access points. Land use and site planning regulations should require consolidated access to parcels or access points being sited on minor streets.
Well-applied access management techniques can improve traffic safety and operations for all road users, not just active travelers.