Traffic calming design standards for new residential streets: A proactive approach

Institute of Transportation Engineers. ITE Journal, Mar 2003 by Womble, Joseph E, Bretherton, Martin Jr

ONE U.S. COUNTY HAS

DEVELOPED A PROACTIVE

APPROACH TO ACHIEVE

TRAFFIC CALMING IN

NEW SUBDIVISIONS THAT

HAVE NOT YET BEEN

BUILT. DEVELOPERS WILL

BE REQUIRED TO INCLUDE

IN THEIR PLANS DESIGN

FEATURES TO ENSURE

REASONABLE SPEEDS ON

NEIGHBORHOOD STREETS,

SUCH AS SPECIFICATIONS

FOR TANGENT LENGTHS

AND CURVES OR DEVICES

SUCH AS TRAFFIC CIRCLES

OR SPEED HUMPS.

PROBLEMS ASSOCIATED WITH residential speeding, both real and perceived, require an inordinate amount of traffic engineers' time and effort in local jurisdictions. Gwinnett County, GA, USA, located in the metropolitan Atlanta area, certainly is no exception. As the population of the county has grown-from 166,808 in 1980 to 352,910 in 1990 to 588,448 in 2000-- so has the number of residential speed complaints.

EARLY TRAFFIC CALMING EFFORTS

Since 1985, Gwinnett County has had an aggressive program of residential speed control. The first effort consisted of selective closures of streets that carried large volumes of traffic taking shortcuts through residential neighborhoods. However, it did not take long to discover that street closures can be quite controversial and that, therefore, the approach should be considered for only the most egregious cases of "cut-through" traffic.

The next effort was a program known as Neighborhood Speed Watch, which sought compliance with residential speed limits through behavior modification brought about by peer pressure, increased awareness and a greater sense of responsibility.1 It was designed specifically for self-contained residential areas, where such an approach is most successful.

Neighborhood Speed Watch worked well for Gwinnett County. Neighborhoods that were in the program for two to three years realized 85th-percentile speed reductions in the range of 11 to 13 miles per hour (mph), which corresponded closely with the results obtained through speed humps. The program did have a serious drawback: To function adequately, it required considerable support from Gwinnett County staff. Neighborhood Speed Watch was eliminated in 1992 during a budget crunch. However, it is interesting to note that speeds in subdivisions that were in the program for two to three years have not returned to their pre-program levels, indicating a lasting modification in behavior.

Following the release of the Institute of Transportation Engineers' first draft on speed hump guidelines, Gwinnett County began an extensive program of retrofitting speed humps on existing streets. Speed humps are installed on a petition basis and capital costs are funded by a special purpose local option sales tax, levied by Gwinnett County to fund transportation and other capital improvements. On a street with 85th-- percentile speeds in excess of 35 mph, the approval of 70 percent of the property owners is required. On a street with 85th-percentile speeds less than 35 mph, 90 percent of the residents must approve. In addition, each property owner on a street with speed humps must pay a special tax assessment of $12 per year in perpetuity for the maintenance of the humps.

Gwinnett County's speed hump program has proven quite popular. Since the inception of the program, 797 humps have been installed in 126 subdivisions.

RESIDENTIAL STREET DESIGN

STANDARDS

Throughout this period, the Gwinnett County Department of Transportation (DOT) has sought to reduce future residential speed problems by taking a proactive role in the development review and rezoning process and by promoting street design layouts that discourage higher speeds. Only limited success in this endeavor has been achieved, as evidenced by the number of speed hump petitions that continues to be received from new subdivisions. One aspect of the problem is the relatively low operating speed required for strong complaints to be voiced. For example, 18 percent of the speed hump petitions received in past months have involved 85th-percentile speeds in the 30-35 mph range, which require approval by 90 percent of area residents.

Residential street design standards typically specify minimum values for geometric design features such as horizontal curves but do not specify maximum values. Gwinnett County design standards are no exception.2 By specifying both maximum and minimum design standards, streets can be designed to operate at speeds that are acceptable in a residential area.

A PROACTIVE APPROACH TO

ACHIEVE TRAFFIC CALMING

Gwinnett County's population now is increasing by more than 20,000 people per year. Therefore, it has been important to take a proactive approach to modify the elements of street layout and design that lead to excessive speed. This has been accomplished only by developing specific design standards and incorporating them into the county Is development regulations. In developing these low-speed design standards, the following factors were considered:

* Once implemented, the standards should result in 85th-percentile speeds in the 25-30 mph range.

* The standards should be easy to understand.

* The standards should offer maximum flexibility to subdivision designers and developers.