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:
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 Office Employment Thoroughfares
Characteristics that influence the choice of design speed in these place types:
Select the lowest practical target speed. As speed increases, so does the safety risk for pedestrians and bicyclists. Studies on this subject have correlated higher speeds with higher fatality rates for pedestrians when struck by vehicles. For place types such as these that have higher pedestrian and bicycle 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, an office employment area would have a target speed of 25 mph, especially if bicycle commuting is to be an attractive choice for employees. The maximum target speed for this thoroughfare is 30 mph. "Group A" cyclists may be comfortable alongside higher speed traffic, but lower speeds are essential in order to encourage the broader population to participate in bicycle commuting.
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 office employment thoroughfares 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. Given the propensity of many commuters to drive as fast as they can on their daily commutes, it is imperative that coordinated signal timing be consistent with the target speed. Commuters are usually intimately familiar with their travel routes, and they will often drive faster than they should if they know that by doing so it will allow them to pass through the next light. Signal coordination that encourages speeding between signals to "hit the green" before it goes red should absolutely be avoided.
For vehicles attempting to gain access from adjacent land uses and crossroads onto the respective thoroughfare (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.
Consider Rush Hour Speed Reductions. Most office employment places experience "rush hour" or "peak hour" pedestrian and bicycle congestion at the beginning and end of the day when employees are arriving or departing. Lunch time can also present peak pedestrian levels if the office employment area has restaurants and commercial services within walking distance. These spikes in pedestrian activity increase the conflicts along the thoroughfare. Low speed vehicular traffic is imperative during these times to provide the safest possible environment for all users. Special speed limits for these time periods should be considered as a method of further prioritizing pedestrians, bicyclists, and transit. Clear and visible signing is vitally important when choosing to reduce speeds in this manner.
Support speed transitions with design elements. It is important to consider and plan for any speed transitions that may exist beyond a given area. Appropriate signing, traffic calming measures, and enforcement are essential to ensure safe speed transitions. This is particularly important in office employment areas where the posted speed increases significantly beyond the place.
Enforcement is an effective measure, but long-term efforts can be costly. Studies have shown that when enforcement measures are removed, vehicles typically resume higher travel speeds. One measure that has proven effective in Minnesota is the use of speed limit signs in conjunction with radar speed measurement displays (see Driver Speed Awareness PDF). Although they are not appropriate in all locations, speed measurement displays are particularly beneficial in areas that are plagued by speeding motorists.
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:
Smaller radii encourage slower speeds. Care should be taken, however, to ensure that school buses and transit buses are accommodated at key locations.
When channelized right turns are chosen, care should be taken to ensure that the geometry of the right turn does not encourage higher speeds.
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