Traffic Management Field Trial

Like most communities, Concord, Calif., receives requests and complaints from residents regarding high travel speeds and traffic volumes on neighborhood streets. In response, the city of Concord completed a Neighborhood Traffic Management Handbook to assist residents in understanding neighborhood traffic issues and defining opportunities to enhance neighborhood livability.

The handbook evolved from the Dana Estates neighborhood outreach program. Intersection bulb-outs at school crossings were universally supported. Some, but not all, residents supported more intrusive measures such as speed humps and traffic circles. Concern focused on device effectiveness and permanency should they be ineffective or unpopular.

To address neighborhood concerns, the city of Concord undertook a program to install temporary speed humps and traffic circles. The devices, made from recycled tires, were designed to be installed and removed in one day. The devices were tested for effectiveness and durability within the city’s maintenance yard before installing them on the street.

Site locations

Two streets in Concord’s Dana Estates neighborhood were identified for testing temporary installations of speed humps and traffic circles. Lynwood Drive was chosen for speed hump installations and Landana Drive was chosen for the traffic circle installation. Both streets are well established with mature landscaping, sidewalks and single-family homes fronting both sides of the street. The roads are 40 ft curb-to-curb and on-street parking is allowed.

Lynwood Drive is a residential street that carries between 2,000-2,500 vehicles per day. Landana Drive, which is residential in nature but functions as a minor arterial, carries between 9,000-12,000 vehicles per day.

Speed humps

The temporary speed hump installations were done in June 2000. Three speed humps—14 ft long and 32 ft wide—were installed on Lynwood Drive. The humps were assembled from 32 pieces measuring 2 x 7 ft. On either side of the speed hump, near the curb, was a tapered edge. The installed humps did not intrude upon the gutter area so as not to block drainage. The humps were shown to be very effective within the test area, reducing the 85th percentile vehicle speed by about 6 mph.

The leading edge of each speed hump began curling within several days of installation. The curling resulted from each section being bowed as vehicles drove over the hump. Temperature changes also were a factor. The city received complaints about the appearance of the humps. In an effort to counteract the curling, the leading edge of one of the three speed humps was glued with a bituminous material. The bituminous material failed to bond the speed hump to the road surface.

Further deterioration of the speed humps occurred within several months of installation. Voids, or “soft spots,” were observed in some of the rubber pieces. These areas of the hump became soft as a result of the material delamination. The supplier inspected the material, confirmed factory defects and replaced the bad sections.

Winter rains resulted in another unexpected problem with the speed hump installations on Lynwood Drive. Sheet flow during rainstorms occurs on Lynwood Drive, and debris caught in the sheet flow lodged under the leading edge of the speed humps, causing further lifting and curling of the hump’s leading edge.

In the months following installation, the city has repaired edge pieces that have torn away from mounting bolts and replaced damaged tape used for pavement markings on the humps. Speed hump sections have continued to deteriorate and have been replaced by the manufacturer. In one instance, the manufacturer sent replacements for speed hump sections that had become defective prior to installation.

As a result, the manufacturer de-signed a U-shaped piece of steel into the leading edge to counteract the curling. Initial findings indicated that this stops the curling. However, the edges do not appear to hold up to high volumes of traffic. As a result of this, the manufacturer provided an end cap to install along the leading edge of each speed hump. The end cap was bolted into the pavement, eliminating the problem.

Traffic circle

A traffic circle was installed at the Landana Drive and Mulberry Drive intersection in December 2000. It replaced a stop sign on Mulberry Drive and eliminated the right-turn-only lane from Landana Drive to Mulberry Drive. The traffic circle installation was intended to reduce vehicle speeds on Landana Drive entering the neighborhoods and provide more effective intersection control at the Landana/Mulberry Drive intersection.

The initial traffic circle combined with pavement striping and advanced signing was expected to be sufficient for traffic control. However, because of the unbalanced traffic flows between Landana and Mulberry, minor adjustments to the island and the intersection approaches were made. Type Q Markers were installed on Landana Drive to define the travelway for drivers who were going on the wrong side of the traffic circle. Observers noted that many elderly drivers had difficulty traversing the circle.

Subsequent to the installation, additional minor adjustments were made to address driver disregard for the intersection yield signs and the higher speeds of entering traffic on Landana Drive.

After a 30-day observation period, the temporary curb was painted yellow to provide improved visibility for the general driving population and additional Q-markers were installed at the intersection approaches and departures to Landana Drive. These markers were necessary to narrow the lanes at the traffic circle to force drivers to slow down when going around a tighter curve.

Commercial truck activity was another operational consideration that became apparent during the 30-day observation period. The circle location was placed within the intersection in such a manner as to slow vehicle traffic. Larger commercial trucks, however, were unable to negotiate the intersection as originally constructed.

The result was that some drivers made use of the rolled curb, sidewalk and lawn areas to maneuver through the intersection. Relocating the center circle several feet resolved this problem.

After relocating the traffic circle, the city’s staff observed that a number of Q-markers were confusing some drivers, especially elderly drivers, with respect to driver right-of-way, channelization and general traffic flow. In response, most of the Q-markers were removed and replaced with temporary curb.

The last remaining issue to resolve was the speed of approaching traffic from Landana Drive through the intersection.

Because of the high speeds, 30 mph and over in a 25-mph zone, and the substantially higher traffic levels, the southbound traffic on Landana was dominating the intersection flows.

In response, an 8-ft-wide speed cushion was installed prior to the intersection. In some instances, neighbors, police and city maintenance crews observed motorists driving on the rolled curb and a portion of the sidewalk to avoid the speed cushion. The cushion is made of a hard rubber material and caused more of a jolt than the speed humps on Lynwood Drive. The speed cushion was subsequently replaced with a less severe hump—like that installed on Lynwood Drive—and signs were installed next to the hump to prevent motorists from driving on the rolled curb and sidewalk to avoid the hump. These changes resulted in more motorists yielding to traffic from Mulberry Drive.

Final analyses

Temporary devices are useful tools in helping the public and staff to evaluate the effectiveness of a proposed traffic-calming program.

The rubber speed humps that were tested need to be redesigned for better durability. The speed cushion was constructed better but causes too much of a jolt to be used on high-volume streets. The installation of both the speed hump and the cushion is time consuming because of the many bolts that need to be set into the pavement.

The rubber curbing used for the traffic circle and median islands is durable, simple to install and easy to relocate.

Project update

The temporary traffic circle and speed humps have been installed for over one year. Based on the research and results of the temporary traffic circle and speed humps, design of a permanent traffic circle is nearly complete and will be constructed in the summer of 2002.

TM+E was informed at press time that based on the finding from the testing mentioned above, the rubber speed humps were recently replaced with asphalt speed humps. The decision was made to use asphalt because it looks better on the street and is more durable. It was noted that the asphalt speed humps also can be installed much quicker and at one-fourth the cost of the rubber ones.     TME