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The Relationship between Street Width and Safety Essay

Design Safety | Width/Safety Relationship | Bullet Point Sheet | Support Resources

Introduction

Over approximately the last 60 years, the design of streets has gone from those designed to accommodate a mix of transportation options, to that designed to carry the maximum number of automobiles as fast as possible. However, not all street types serve the same purposes. Highways, freeways and the Interstate Highway System are designed for the sole purpose of maximizing the speed of travel and convenience of automobile use. Residential design must be different to accommodate the character of the street. Unfortunately, streets in residential neighborhoods are now being designed using similar standards, yielding a situation that is not only inconvenient and inefficient, but also very dangerous. Streets must be designed to maximize overall safety.

Municipal decision makers need to take responsibility for the overall safety of the streets in their community. According to the House Committee on Public Works (U.S. Congress) (as found in A Policy on Geometric Design of Highways and Streets: 2001 by AASHTO):

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“Whose responsibility is it to see that maximum safety is incorporated into our motor vehicle transportation system? On this, the subcommittee is adamant. It is the responsibility of Government and specifically those agencies that, by law, have been given that mandate. This responsibility begins with the Congress and flows through the Department of Transportation, its Federal Highway Administration, the State Highway Departments and safety agencies, and the street and highway units of counties, townships, cities, and towns. There is no retreating from this mandate, either in letter or in spirit.”

Safety

The very nature of highways dictates wider paved surfaces. These thoroughfares are designed with the sole purpose of maximizing automobile speed and convenience. As the design speed of a road increases, naturally, the width of the paved surface will also increase. Wide lanes allow drivers to have adequate room to make adjustments and decisions while traveling at high speeds, and therefore decrease the likelihood of an accident. Other design features that add to highway safety include limited access points, a minimum number of intersections, prohibition of pedestrians and bicycles, minimum speed limits, and maximum sight distances. The Interstate Highway System was designed to move as many vehicles as possible as quickly as possible. It was not designed to include pedestrian or bicycle traffic. In fact, many highways strictly forbid non-motorized vehicular travel all together.

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With the introduction of pedestrians, on-street parking, bicycles and other attributes of residential neighborhoods, highway-like standards are no longer the safest, and are even frequently very dangerous. Unfortunately, it has become common practice to implement standards similar to those used for highways to design and build our residential streets. Using highway standards in a residential setting increases driving speed. Faster speeds increase danger by intensifying impact. The harder a person is hit, the more likely it is that he/she will be seriously hurt or killed. In addition, the faster a vehicle is moving, the greater the distance it needs to stop. Therefore, at higher speeds, a driver is less likely to be able to avoid a serious accident.

Streets in residential neighborhoods should be designed with the understanding that pedestrians and bicyclists as well as automobiles use them. Thus, vehicular speeds must be decreased to minimize the likelihood and severity of pedestrian/bicycle and automobile conflict. However, merely posting a residential street with low speed limits does little to insure that vehicles will travel at safe speeds.

When it comes to residential streets, narrower streets are better. Narrow streets slow traffic by increasing drivers’ perception of impediments to motion. If a driver perceives more obstructions, he/she is more likely to drive at slower speeds to avoid potential conflicts. The American Association of State Highway Transportation Officials (AASHTO) asserts in, A Policy on Geometric Design of Highways and Streets: 2001, that residential streets are typically associated with short trips and are commonly used as a means of accessing property, so there is no need for high-speed travel.

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Wide residential streets contribute to the number of automobile accidents. Swift & Associates conducted a study entitled, Residential Street Typology and Injury Accident Frequency (March 31, 1998), that looked at over 20,000 accidents in the City of Longmont, Colorado. According to that study: “…The most significant relationship to injury accidents were found to be street width and street curvature. The analysis illustrates that as street width widens, accidents per mile per year increase exponentially, and the safest residential street width is 24 feet (curb face).” Narrower roads decrease the likelihood of automobile related accidents. Furthermore, according to the Swift & Associates study: “…Since municipal code generally mandates a 36 foot wide residential street (planned unit development may be an exception), the results from this study indicate that current street design standards are directly contributing to automobile accidents.”

Not only do excessively wide residential streets contribute to the number of accidents, they also add to the severity of accidents. Wide residential streets encourage speeding and in doing so, increase the potential severity of accidents. According to an article by John Anderson entitled, “Framing the Debate on Streets and Public Safety” (New Urban News, July/August 2000): “…wide streets encourage speeding and increase the risk of traffic accident injuries and fatalities.”

Mary Stalker, John Anderson, and Tom DiGiovanni do a great job of explaining the correlations between road width, traffic speed and the severity of injury in the event of an accident. In their article entitled, “Streets and Fire Trucks: Designing Streets for Emergency Response and Neighborhood Safety” (New Urbanism: Comprehensive Report & Best Practices Guide, 2001), they state the following:

“A 24-26 ft. (curb-to-curb) street with parking both sides is considered ideal for walkable streets because it slows traffic (traffic calming through street geometry). The wider the street, the more likely cars are to speed—and with faster speeds, accidents become more likely, and more deadly (pedestrians hit by vehicle traveling 15 mph have a 96 percent chance of survival; at 40 mph, survival chance drops to 17 percent). Walkers feel most comfortable when vehicle speeds are kept to 10-25 mph, because slower traffic means safer streets!”

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The Institute of Transportation Engineers (ITE) provides additional information that supports this relationship in Traditional Neighborhood Development Street Design Guidelines. They assert that as vehicular speed increases, the likelihood of fatality increases when a vehicle strikes a person. The effects of vehicular speed on the likelihood of severe injury or death in the case of an accident are evident.

From the standpoint of overall safety, wide pavement widths are safer for non-pedestrian highways, freeways, and the Interstate Highway System. However, when dealing with streets used by pedestrians, narrower road widths are the safest. Narrow streets slow traffic speeds, which in turn, decreases the likelihood of an accident while simultaneously decreasing the potential severity of injury if an accident does occur. This should be sufficient reason for a community’s elected of appointed officials to support and mandate the use of narrower residential streets.

Conclusion

Perhaps Peter Katz said it best (The New Urbanism: Toward an Architecture of Community, 1994), “Streets should be safe, interesting and comfortable for pedestrians.” However, according to Andres Duany, Elizabeth Plater-Zyberk and Jeff Speck (Suburban Nation: The Rise of Sprawl and the Decline of the American Dream, 2000), “Streets that once served vehicles and people equitably are now designed for the sole purpose of moving vehicles through them as quickly as possible. They have become, in effect, traffic sewers.” In order to maximize overall safety, transportation planners and engineers need to reassess the current standards for road design and construction to make them more appropriate for their intended use. According to AASHTO, “The first step in the design process is to define the function that the facility is to serve.” Highways should continue to be designed solely for automobiles. However, the standards used for residential streets should be changed so they are designed to accommodate all modes of travel, from pedestrians to bicycles to automobiles. Overall safety will remain compromised until streets are, once again, designed for the users they are intended to serve.

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