Sunday, October 9, 2011

New Design Standards For Neo-traditional and Low Speed Neighborhood Streets

By C. Edward Walter and Andrew P. O’Brien

Speeding traffic through residential neighborhoods is the number one concern of most community associations. Judging from the popularity of recent I TE traffic calming programs and publications is rapidly becoming the number one concern of traffic engineers as well.
For more than nine years Howard County Maryland has been calming its suburban residential streets. These streets have a speed limit of 25 mph, which few drivers obey. Speed studies consistently reveal eighty-fifth percentile speeds of 37 to 40 mph on uncalmed streets. These high speeds increase accident severity when they occur. Pedestrians are much more apt to be seriously or fatally injured if struck at 40 mph compared to 25 mph accidents. There’s a general feeling that high speeds degrade a neighborhood; making it a less desirable place to live. Sometimes homeowners deliberately drive at or below the speed limit in an attempt to slow following drivers. This is generally a futile activity, which only frustrates other drivers and may contribute to road rage.
There are a number of reasons for speeding in residential areas. Many drivers are in a hurry. Some drive distracted with coffee in hand or applying makeup or even shaving. Sometimes neighborhood streets are a cut through path for drivers trying to avoid arterial congestion. Generally the real culprit in residential speeding is the use of improper design standards for residential streets.
Residential streets are typically designed using minimum geometric standards of 30 - 35 mph despite planned operating speeds of 25 mph. Such over design contributes to speeding. If the minimum curve radius is 300 feet for a 30 mph design standard, the use of a longer radius increases the theoretical design speed of the street and contributes to even higher speeds.
Street width is another important factor in residential speeding. Wide streets contribute to speeding. Fortunately street width standards in recent years have been revised downward. Widths of 38 to 40 feet curb to curb have been revised downward to 28 to 34 feet. Yet even these widths contribute to residential speeding.
The main principles of traffic calming introduce geometric changes or visual changes to reduce speed. Introducing speed humps, roundabouts, road narrowing, mid-block chokers, chicanes, semi-diverters, gateways, pavement texture, and other traffic calming tools modifies existing streets. Such calming measures may be introduced on individual streets or through entire neighborhoods.

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