December 4, 2002

Ms Sue Tenenbaum
Traffic Circle Review Committee Coordinator
Barcroft School and Civic League
Dear Ms Tenenbaum:

This is in response to your email of Novemeber 21st concerning the traffic circles proposed for South Wakefield Street in Barcroft. In this message, you asked several questions about various traffic measures and the process.

Concerning speed and safety, you asked about the effectiveness of allway stop signs, speed humps and traffic circles.

Allway Stop Signs: Stop signs are not a traffic calming measure and are used to assign the right of way at intersections. For low volume streets we typically have in residential neighborhoods, allway stop signs tend to reduce speeds within a zone of influence (50 to 75 feet) near the intersection. Mid-block speeds tend to be unchanged, and some studies have actually shown an increase in mid-block travel speeds. We have documented this on a section of North Manchester Street south of Wilson Boulevard where three-way stop signs were installed under the County's warrants about two years ago.

Under current County policy, an allway stop sign could be installed at the intersection of South Wakefield Street and 8th Street South. While this may slow traffic near the stop sign, it would likely have little or no effect on speeds farther away.

Stop sign compliance is a problem at allway stop signs, and becomes more so when the traffic volumes on the streets approaching the stop signs are unbalanced. The traffic volume on Wakefield Street is 1800 vpd*, while 8th Street carries 700 vpd. With volumes being unbalanced as they are, a motorist traveling Wakefield Street would likely not encounter a vehicle on the cross street. Vehicles on the cross streets are the most effective deterent to motorists running stop signs. We have documented cases in Arlington where more than 40% of motorists roll through or do not stop where allway stop signs have been installed. We are currently in the process of video taping stop sign compliance at several intersections where allway stop signs have been installed, and will have this report completed next spring.

So, stop signs are not the panacea some folks think they are. They do not control speeding away from the stop sign and there are serious stop sign compliance issues that can make an intersection less safe.

Speed Humps: Speed humps are very effective devices for slowing traffic along the entire stretch of roadway, since we space them about 400 feet apart. Where we have installed speed humps, we are seeing a typical reduction in travel speeds of 4 to 6 mph. We sometimes experience a reduction in volume as traffic seeks to avoid the humps.

Traffic Circles: The small mini-traffic circle has been used in Arlington since 1988 to slow traffic on neighborhood streets. These devices work best when installed as a series. Data collected after installation has shown a reduction in travel speed of 2 to 5 mph, depending on the diameter of the circle. Seattle, Washington originally constructed mini traffic circles to reduce crashes, with speed reduction a secondary benefit. The number and severity of injuries is also reduced because of the lower speeds of the vehicles approaching the intersection. We have also seen a reduction in crashes at intersections where circles have been installed, especially on the higher volume streets such as South Dinwiddie Street (near Wakefield HS) and North Fillmore Street (north of Pershing Drive).

Use of Traffic Circles Near Schools: We have not installed traffic circles in the County on streets adjacent to elementary schools, however, there is a large circle on S. Dinwiddie Street in front of Wakefield HS. In researching the literature, I could find no reference for not using traffic circles near schools. In the past we have installed traffic calming measures near elementary schools, but the measures employed were measures like raised crosswalks, speed humps and gateways. The NTC traffic calming guide does not address circles near schools, and it is something that we have not dealt with in the past. However, if the crossing guards have reservations about the use of circles on Wakefield Street near the school, I believe we should defer to their concerns about their use and design.

Procedures for changing approved NTC plan: The NTC plan for Barcroft approved by the County Board in January 2002 and funded the Wakefiled Street portion at $103,000. Since the program is relatively new, we have not dealt with a situation where we have gone through the process including the petition process and have the community change its mind about certain measures.

I would think the process from this point on would go as follows:

1. Design of circles is put on-hold.

2. Staff works with Wakefield St. residents and school officials early next year on alternative measures.

3. If alternative measures are determined by working group, these are presented to civic association and NTCC.

4. New petition is circulated in the affected area to eliminate circles and propose alternative plan. If approved by at least 60% of affected area, new plan adopted.

5. If new plan cost exceeds $103,000, the NTCC must recommend additional funding for project. If no additional funding needed, NTCC chair would inform County Board of change in plan and indicate community support levels.

6. County Board may consider additional funding request at next NTCC funding round. If no additional funds are needed, County Board informed through memo from County Manager.

I hope this is helpful in your discussions. In January, we will be willing to meet to begin a substantive discussion on the Wakefield Street plan.

Sincerely,

Jeffrey Sikes
Neighborhood Traffic Calming Coordinator

* vpd = vehicles per day

This page was last revised on: December 6, 2002