"Cambridge is very proud of the traffic calming measures accomplished thus far, as well as the projects that are now underway. This recognition celebrates the much needed traffic improvements in our community."
- Mayor Duehay

Speeding traffic on neighborhood streets affects the safety and quality of residents’ lives. While driver behavior is a significant part of the problem, speeding is also encouraged by the design of streets because they were built to allow for faster travel than is safe. As a result, traditional methods of controlling traffic speeds - signage, signalization, and enforcement - are not able to manage the problem effectively.

Over the past 25 years, engineers, planners, and communities have been developing ways of retrofitting streets to make the design match the desired speed; this is known as traffic calming. Originating primarily in Western Europe, these new design techniques were successful and popular and spread to Australia, Canada, and the United States.

The City of Cambridge started using traffic calming techniques around 1995, incorporating various measures on a limited scale during street reconstruction projects. Given the success of these measures, which received strong support from residents, the city made a commitment to implementing traffic calming throughout the city.

In 1997, the city executed its most extensive traffic calming project on Berkshire and York streets. Undertaken to reduce speeding and improve safety on whole streets, this project marked the real beginning of the Traffic Calming Program for Cambridge.

Berkshire and York Streets Design

Berkshire and York streets are bordered by a playing field, an elementary school, a branch library, and a youth center on one side and houses on the other. Residents had complained about speeding vehicles for a long time, and there had been several accidents involving children crossing the streets. Police checks found that speeding and running the stop sign at the intersection of York and Plymouth streets were prevalent. There also was concern in the neighborhood that Berkshire Street was being used as an alternative route for commuters during the p.m. peak hours from 4:00 until 6:00 p.m. To address these concerns, the city worked with residents to design a comprehensive traffic calming plan for Berkshire and York streets. Construction was completed in the summer of 1997.

The traffic calming measures used on this project include:

Curb extensions at many intersections (See Diagram 1). By tightening up intersections, curb extensions reduce the length of crosswalks, slow vehicles by requiring tighter turns, reduce the amount of pavement, increase the amount of sidewalk, and change the emphasis of an intersection away from motorized vehicles. Curb extensions also prevent cars from parking illegally at corners, thus improving sight lines for cyclists, drivers, and pedestrians.

A raised crosswalk across Berkshire Street at Hardwick Street (See Diagram 2). This improves pedestrian safety in a number of ways. It encourages the use of the crosswalk by providing a strong visual cue and by not requiring the pedestrian to step down to the level of the roadway and then back up on the other side. It also slows vehicles by acting as a speed hump with a broader, flatter area to traverse than the old speed bump. The approach slopes of the speed hump were constructed at 8 percent, i.e., degree of steepness of transition from the roadway surface to full height of the raised device.

Raised intersections (See Diagram 3) at the following intersections:

• York and Hamlin streets.
• Berkshire and Marcella streets.

Raised intersections improve pedestrian safety in the same ways as raised crosswalks. They encourage the use of the crosswalks by providing a strong visual cue; do not require the pedestrian to step down to the level of the roadway and then back up on the other side; and slow vehicles by acting as a speed hump. They also send a message to drivers entering the corridor to be more aware of non-motorized users. The approach slopes of the raised intersections were also constructed at the same degree of steepness as the raised sidewalks, 8 percent.

4. A chicane, which is a lateral shifting of the roadway, on Berkshire Street between York and Hardwick streets (See Diagram 4). The chicane reduces the width of Berkshire Street by a total of 14 feet - 7 feet on each side. Chicanes are designed to reduce speeds by introducing a shift in the roadway alignment.

5. Use of zebra crosswalk markings throughout the project (See Diagram 1). Zebra crosswalk markings emphasize the pedestrian crossing to both drivers and pedestrians.

6. Relocation of the fence openings around the playing field to line up with the enhanced pedestrian crossings. This encourages pedestrians, particularly children, to cross the street where it is safest to do so.

The Berkshire and York streets improvements were constructed as a comprehensive traffic calming plan. The various measures work together to change the nature of the roadways and to slow down vehicles.

All of the vertical changes, i.e., the raised crosswalk and the raised intersections, were constructed with concrete pavers. The pavers replicate the look and feel of brick. Pavers were used instead of brick for their durability under traffic and slip-resistant finish. The change in color and texture from the asphalt street enhances the overall effectiveness of the improvements. As previously indicated, approach slopes to the vertical changes were constructed at 8 percent. The dramatic pavement markings on the approach slopes (See Diagrams 2 and 3) provide a strong visual cue to the driver that the roadway is changing elevation. All of the pavement markings use inlay tape which is highly visible, reflective, slip-resistant, and long-lasting.

Results

As part of the ongoing evaluation of this project, before and after speed studies were conducted. Before the improvements, the 85th percentile speed on Berkshire Street was 30 miles per hour (mph). After the improvements, the 85th percentile speed was reduced to 21 mph at the vertical traffic calming devices and to 24 mph in between. The 85th percentile speed is the speed under which 85 percent of the vehicles travel and is the industry standard for design purposes and speed studies.

Before the improvements were made, only 41 percent of vehicles were obeying the 25 mph speed limit. After the improvements, 95 percent of vehicles obey the speed limit.

Decreasing vehicle speeds is a great measure of project success because such reductions lessen the risk and severity of crashes.

Contact: Kathy Watkins, Traffic Calming Project Manager, Community Development Department, Cambridge, 617/349-4655.

Table of Contents

The United States Conference of Mayors

J. Thomas Cochran, Executive Director
1620 Eye Street, NW, Washington, DC 20006
Telephone (202) 293-7330, FAX (202) 293-2352

Copyright ©1996, U.S. Conference of Mayors, All rights reserved.