• Image 05
  • Image 04
  • Image 01
  • Image 02
  • Image 03

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 Residential Neighborhoods

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

  • Design speed, target speed, and posted speed are equal
  • Significant pedestrian and bicycle presence
  • Significant transit presence
  • Quality of life for local residents isa key consideration

Select the lowest practical target speed.Doing so creates the safest environment for pedestrians, provides easier access to/from abutting land uses, and eases transition between modes of travel.Residential areasare often popular bicycling places due to the lower speeds and transit provisions (transit users often use bicycles to negotiate longer transfers and/or to get from home to the transit stop). For great streets, it is imperative that the choice ofdesign speed reflect the vision for the desired environment. Ideally, a residential neighborhood would have a target speed of 15 mph, maximum 25 mph.

Once the target speed is set (assuming that the aforementioned recommendation of equating design speed and target speed is adhered to), it is critically important to design and develop all of the design elements of the arterial, e.g. the curvature of the road or the width of the lanes, according to the posted speed. The target speed limit will become nothing more than a number on a sign if the controlling design features of the arterial allow and promote travel at higher speeds. Horizontal curvature, lane width, horizontal clearance, median type and width, and many other features must be chosen to be consistent with the posted speed limit.

Pedestrian safety graph
Credit: CH2M HILL

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 place types such as this that have higher pedestrian activity levels, slower speeds go a long way to increase pedestrian safety along the street. Traffic calming measures such as speed bumps or rumble strips can be effective ways to promote the desired lower speeds. Appropriate signing that alerts drivers to the presence of such measure is important, especially at night when it may be more difficult for drivers to recognize them.

Chicane
Credit: Dan Burden

Many of our residential streets are linear; that is, they are often very straight without much roadway curvature. Streets that are straight can sometimes encourage speeds above the slower speeds desired in residential place types. To combat this, the AASHTO Guide for the Planning Design, and Operation of Pedestrian Facilities recommend use of "chicanes", or curb extensions that can combat the straight-line, wide-open feel often found on residential streets. Neighborhood traffic circles (different than modern roundabouts) can be used for similar purposes at intersections.

Traffic circle
Credit: css.org

There are a variety of intersection design treatments that can help contribute to lower speed environments. Neighborhood traffic circles, as shown in the imageon the right,(not to be confused with modern roundabouts) are circlesbuilt in the middle of intersections to slow vehicular speeds down through the intersection.Traffic circles are often landscaped, and therefore also aesthetically enhance the streetscape.

Traffic circlesprevent through-traffic on the street from driving straight through the intersection by forcing drivers to alter their path of travel around the circle.

When considering neighborhood traffic circles as an intersection design treatment, the following points should be kept in mind:

  • Landscaping, public art, water fountains, and other treatments can be used in the center of the traffic circle to add aesthetic value to the street. While they certainly can be aesthetically very pleasing, the height of such treatments can impede visibility for pedestrians at the intersection as they try to cross the various legs of the street.
  • Nighttime lighting in residential neighborhoods is often intentionally darker than other arterial streets to accommodate local residents. This can make it difficult to clearly see neighborhood traffic circles as drivers approach the intersection. Raised cobble stone strips or other such raised pavement treatments in advance of the intersection can be an effective way to alert drivers to the approaching intersection treatment. Advanced signing located in areas of clear visibility are also important in this regard.
  • Intersections with high left turn volumes should not consider neighborhood traffic circles due to the awkward geometry constraining the left turn movements. Left turning vehicles tend to cut through the left side instead of driving around the circle.

Modern roundabouts can also be an effective traffic calming method for residential streets. Modern roundabouts are similar to neighborhood traffic circles only in that they also have a central island in the middle of the intersection. They differ substantially, however, on the size of that central island and in the curvature of the intersection approaches and intersection corners. These features are designed to promote the desired speed through the roundabout, in this case lower speeds. See Intersections for more details.

When considering traffic calming techniques, coordination with emergency services is very important. Traffic circles, speed bumps, chicanes, etc. can make it difficult for large vehicles such as fire trucks when responding to emergency situations. Location and design of these measures should consider emergency access.Traffic calming is encouraged, we simply need to coordinate implementation with emergency service providers.

If lower speeds are used in these place types, it is easier for drivers to perceive conflicts on the road ahead and react accordingly. The lower the speed, the less time and distance required to stop or slow down to avoid a potential conflict. Conflicts on the street are numerous in these place types, and they can be caused by vehicles entering or exiting the street from adjacent access points; by pedestrians unexpectedly entering the traveled way; by vehicles stopping to park or pulling out of parking stalls; buses pulling over at a stop or pulling out from a stop; or by other vehicles unexpectedly changing lanes in congested conditions. Again, lower speeds allow drivers to more safely navigate the myriad of conflicts that may present themselves at any given moment along streets in these place types.

There is often a misperception that slow speeds result in slower travel times along a given street. The travel time along thoroughfares, however, can only be as fast as the intersections (particularly the signalized intersections) allow. High posted speed limits will do nothing to improve travel time if there is significant delay experienced at the intersections. In fact, slower speeds along a streetcan 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 respectivestreet (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 or "gap" is acceptable in order to safely navigate to and from the street. As speed increases, the number of acceptable gaps decreases and it becomes increasingly more difficult for drivers to identify safe gaps. Selecting the lowest practical design speed for these place types will maximize the ability of drivers to effectively assess gap acceptability, and as a result more safely enter and exit themain traffic stream.

There is a large amount of research that has been recently conducted on 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 lanes of 10', 11', and 12' wide. In other words, lane width has little effect on substantive safety in low-speed environments. Sinceresidential streets should typically be designed for speeds at or below 35 mph, this suggests that we have a fairly wide range of lane widths to choose from. Generally speaking, narrower lanes will encourage lower travel speeds and are therefore recommended for these place types. The site-specific characteristics will influence the choice between 10', 11', and 12' lanes. Narrower lanes also leave more r/w 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.