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Design Speed

Article

Design speed is the rate of travel for which the physical characteristics of a roadway are designed. The design speed for a given roadway plays a large role in determining the scale and design of roadway characteristics. For example, if a design speed of 35 mph is chosen for a given roadway, all aspects of its design, such as roadway curvature, lane width and intersection elements will safely accommodate vehicles traveling at 35 mph. Roadside design elements vary greatly with design speed, as fixed objects along the streets present a greater substantive safety risk at higher speeds.

Before discussing design speed, it may be useful to introduce and explain a few related topics:

  • Operating Speed: a measure of the speed at which most drivers actually travel on a given arterial section under free flow conditions (often equated to the 85th percentile speed of traffic observed under free flow conditions). On urban and suburban arterial streets, operating speed is heavily influenced by the presence, spacing, and timing of traffic signals.
  • Target Speed: the speed at which drivers should travel on a given arterial section. Ideally, a facility's target speed and posted speed should be the same.
  • Posted Speed: the upper speed limit for a given arterial section; often commensurate with target speed. The posted speed often represents the desired target speed.

Designers and engineers often choose a design speed that is higher than the posted/target speed, which encourages vehicles to travel at speeds higher than the target speed, especially along lower speed corridors. A facility's design speed and target speed should be equal, to keep vehicular speeds at or below the desired target speed. All elements of the streetscape should be designed to support the target speed for the corridor.

In a multi-modal environment with significant pedestrian presence, it is essential to provide adequate vehicular stopping sight distance and intersection sight distance. It is good practice to use a relatively low design speed (e.g. 30 mph) but provide the equivalent of 40 mph of sight distance.

Functional classification is traditionally used to determine the target speed for a given arterial street. Although roadway planners and designers should consider functional class when selecting the facility's posted speed, the characteristics of each individual place should be the primary consideration used in choosing a target speed. A keen awareness of an area's unique characteristics will prevent the misapplication of broad standards that may be inappropriate for the place.

Design Speed for Neighborhood Shop Thoroughfares

Characteristics that influence the choice of design speed for these place types:

  • Design speed, target speed, and posted speed are the same; and
  • There is a significant pedestrian presence.

<insert pedsafety.bmp, source = AASHTO> Select the lowest practical target speed. As speed increases, so does the safety risk for pedestrians. Studies on this subject have correlated higher speeds with higher fatality rates for pedestrians when struck by vehicles. For neighborhood shop thoroughfares that have higher pedestrian activity levels, reducing speeds is an effective way to improve safety. Selecting the lowest practical target speed creates the safest environment for pedestrians, provides easier access to/from abutting land uses, and eases the transition between modes of travel. Ideally, neighborhood shop thoroughfares would have a target/design speed of 20 mph, maximum 25 mph.

Lower speeds also make it easier for drivers to perceive conflicts on the road ahead and react accordingly. Drivers require less time and shorter distances to stop or slow down to avoid conflicts in low-speed environments. Conflicts on the street are numerous along neighborhood shops streets due to vehicles entering or exiting the street from adjacent access points; pedestrians unexpectedly entering the traveled way; vehicles stopping to park or pulling out of parking stalls; buses pulling over at a stop or pulling out from a stop; and other vehicles unexpectedly changing lanes in congested conditions. 

There is often a misperception that slow speeds result in slower travel times along a given arterial street. The travel time along arterials, however, can only be as fast as the intersections (particularly the signalized intersections) allow.  High posted speed limits will do nothing to improve arterial travel time if there is significant delay experienced at the intersections. In fact, slower speeds along an arterial can contribute to improved overall travel times by allowing more time for better progression and coordination between signals.

For vehicles attempting to gain access from adjacent land uses and crossroads onto the respective arterial (or vice versa), identifying an opening in the traffic stream to safely enter, exit, or cross (commonly referred to as gap selection) is of paramount importance. Drivers must be able to accurately assess whether an opening is acceptable in order to safely navigate to and from the arterial. As speed increases, the number of acceptable gaps decreases and it becomes increasingly difficult for drivers to identify safe gaps. Selecting the lowest practical design speed for a corridor will maximize the ability of drivers to effectively assess gap acceptability, and as a result, safely enter and exit the arterial traffic stream.

Design for the target speed. Once the target speed is set (and consequently the design speed and posted speed), controlling roadway elements must be carefully designed to support travel at the desired speed. The target speed limit will become meaningless if lane widths, horizontal clearance, median type and width, and other features are inconsistent with the posted speed limit. ITE's Context Sensitive Solutions in Designing Major Urban Thoroughfares for Walkable Communities identifies the following design elements that should be considered when lower speeds are desired:

  • Select narrow lane widths. Selecting narrower lanes helps to reduce travel speeds, and conversely, lanes that are excessively wide contribute to higher speeds. There is a growing body of research regarding the correlation between lane width, speed, and substantive safety. The latest research suggests that for travel speeds equal to or less than 35 mph, there is no difference in substantive safety performance between lane widths of 10', 11', and 12'. In other words, lane width has little effect on substantive safety in low-speed environments, such as neighborhood shop streets. Site-specific characteristics will influence the decision between 10', 11', and 12' lanes. Additionally, narrower lanes also leave more right-of-way available for the areas "beyond the pavement", such as sidewalks, tree plantings, building frontages, etc. Converting a 4-lane section with 12-foot-wide lanes to 10-foot-wide lanes will provide a net gain of 8' in additional right-of-way for other uses.
  • Eliminate super-elevation in horizontal curves. If horizontal curves are present along the thoroughfare, super-elevation, or banking of the roadway through the turn, is not recommended as it can encourage higher speeds.
  • Eliminate shoulders. Here again, the narrower travel way can have a traffic calming effect, which should provide a safe environment for bicycles to share the travel lane with vehicles. Right-of-way in neighborhood shop areas, beyond the travel lanes, is best used for improved facilities for pedestrians, transit and bicycles.
  • Include on-street parking. The presence of on-street parking has economic benefits in neighborhood shop areas as well as traffic calming effects. Parking movements tend to calm traffic and the presence of parking cars, once again, has a narrowing effect on the travel way, ultimately contributing to slower speeds. On-street parking also provides a buffer between vehicular traffic and the pedestrian realm.
  • Use curb extensions when appropriate. Particularly at pedestrian crossings and intersections, curb extensions as shown at right, can also contribute to narrower streets and slower speeds. Curb extensions also increase visibility between pedestrians and vehicular traffic. Coordination with emergency services is important to ensure that curb extension design accommodates larger vehicles such as fire trucks.
  • Minimize intersection curb return radii. The size of the radii for the curb returns at intersections has a direct impact on vehicular travel speed in neighborhood shop areas.

    Smaller radii encourage slower speeds.

    When channelized right turns are chosen, care should be taken to ensure that the geometry of the right turn does not encourage higher speeds.

  • Increase awareness of bicycles and pedestrians. Pavement treatments that delineate and highlight sidewalks, crosswalks and bicycle lanes, increase driver awareness of their presence. Textured materials and pavers, colored concrete, and pronounced striping can effectively highlight pedestrian, bicycle and transit facilities and consequently discourage higher travel speeds.
  • Post clear signage. Clear and concise signs along the street are vital in communicating the speed limit to vehicular drivers.  See the signing section of this manual for more information.