Inter-Departmental Correspondence

Department of Public Works



May 15, 2002


June 4, 2002



Honorable Board of Supervisors


Neil R. Cullen, Director of Public Works


Installation of "Test" Traffic Calming Devices



Determine if the Department of Public Works should install two types of traffic calming devices on different streets to determine:



their effectiveness in physically reducing the speed of traffic; and



if the devices meet the expectations of the property owners in reducing the speed of traffic as compared to any inconveniences that the devices may create.


Key Facts

The California Highway Patrol (CHP) - the agency with primary responsibility for enforcing the provisions of the California Vehicle Code on roads in the unincorporated area - and the Department continue to receive requests from property owners and property owner groups requesting traffic control or traffic calming devices to slow the speed of traffic on their streets.


Limited speed test evaluations by the Department, the CHP and the Sheriff indicate that speed humps and dips would need to be relatively abrupt if they are to be used to control the speed of all the motorists traveling on a given road.


A review of the data and comments relative to traffic calming devices as published on the Internet indicates that there are some issues associated with the installation of speed control traffic calming devices for persons with back problems or other physically disabling conditions.


Our past experience in North Fair Oaks and in West Menlo Park indicates that traffic calming devices may be more of a psychological deterrent to speeding rather than a physical deterrent.



Previous Installations of Traffic Calming Devices

The attached list is a summary of some of the traffic calming devices that are suggested to be used to either reduce the speed of traffic or reduce the amount of cut through traffic in a residential neighborhood. The device to be used depends of the physical conditions of the particular street and what is proposed to be accomplished. As an example, your Board approved the use of traffic circles on Fair Oaks Avenue to eliminate the appearance of a long straight road that was conducive to speeding. The amount of traffic was also perceived to be less by the property owners even though before and after traffic counts were essentially the same.


Your Board also approved the construction of entry gates on Cloud and Sherman Avenues in the West Menlo Park area. The gates remain in place but we have not constructed additional gates as a survey of the property owners indicated that there was not an overall consensus that the gates were something the property owners on the other streets desired.


Smaller street width standards have also been approved which in of themselves may not be a traffic calming device, but which can effectively result in street narrowing when vehicles are parked on both sides of the street. An example of where traffic is slowed by cars parking on relatively narrow streets is Stanford Avenue in Fair Oaks. However, we continue to get requests for stop signs as the speed of traffic on these streets is perceived as too high.


Speed Limits

The California Vehicle Code provides for two speed limits - prima facie (a posted speed limit either prescribed by the Vehicle Code or established by local ordinance); or the basic speed limit (a motorist travels at a speed reasonable for the conditions).


Speed limits different from the prima facie speed limits are determined based on an engineering and traffic study. The studies include speed surveys to determine the speed of traffic at the 85th percentile - i.e., 85% of the motorists are traveling at or below a given speed. This methodology acknowledges that 15% of the motorists are comfortable traveling at a higher rate of speed and will probably continue to travel at a higher than posted speed. Setting speed limits lower than the 85th percentile can constitute a speed trap as defined by the Vehicle Code and preclude the use of radar to enforce the speed; or a large percentage of motorists may drive faster than the posted speed limit as they are comfortable driving at the higher rate of speed (i.e., driving based on the basic speed law concept).


Stop Signs

Stop signs are not included in the category of traffic calming devices as Section 21400 of the California Vehicle Code provides for "...rules and regulations prescribing uniform standards for all traffic controls devices placed pursuant to the provisions of this code, including but not limited to stop signs, yield right-of-way signs, speed restriction signs, ...". The Department follows the regulations in determining and recommending where stop signs should be installed.


Allowed Use of Traffic Calming Devices

Section 440 of the Vehicle Code also provides that "official traffic control device(s) does not include islands, curbs, or other roadway design features. This language allows for the local determination of traffic calming devices as traffic circles, chicanes, speed humps and dips are roadway design features.


Evaluation of Traffic Calming Devices

We constructed three speed control devices (shallow dip, speed hump, double speed hump) at the Half Moon Bay Airport and asked the Sheriff and the CHP to traverse these devices at various speeds up to 70 mph to determine if either the discomfort level increased or if loss of control of the vehicle occurred. Both the Sheriff and the CHP were able to traverse the three devices at high speeds with little or no discomfort to the driver and with no loss of control. We attribute this to the improved factory suspensions that vehicles are being equipped with.


The CHP also evaluated the valley gutter that we installed across Canyon Road off of Cordilleras Road near Redwood City. The valley gutter is deeper than the shallow speed dip and shorter in width. The CHP, while not able to traverse the valley gutter at high speeds due to the configuration of the road, did experience discomfort at speeds greater than 25 mph.


Conclusions Reached

We concluded, based on the results of these limited evaluations, that the speed control devices that are currently in use may not slow down those drivers that inherently drive at higher than the posted speeds (i.e. the 15% traveling at a speed higher than the 85th percentile); and that more abrupt devices similar to the valley gutter on Canyon Road would have to be installed if the intent is to reduce the speed of all drivers. However, the more abrupt speed control devices may cause greater discomfort for the majority of drivers who are traveling at or just above the posted speed (i.e., traveling at or below the 85th percentile).


Installation of "Test" Traffic Calming Devices

The CHP has requested that we install two speed humps on Hillside Drive in the Burlingame Hills and a valley gutter on Blenheim Avenue in North Fair Oaks, as groups of residents in each area have expressed concern as to the speed on these two streets.


Both streets have a mix of neighborhood traffic as well as through traffic. Hillside Drive in Burlingame Hills is used by both the residents and non-residents to access Skyline Boulevard and Interstate 280; and the residents on Blenheim complain that motorists from a motorcycle shop or from other businesses on El Camino Real travel down their street at unreasonable speeds.


We propose, if your Board determines that the devices should be installed, that speed surveys and traffic counts after the traffic control devices are in for at least 3 months be conducted to determine the speed of traffic and the volume of traffic. We also propose that a survey of the property owners be conducted after the 3 month period to determine if they believe the speed of traffic has been reduced and if they want the speed control devices to remain in place.


We can concurrently develop criteria to be considered by your Board before traffic calming devices would be considered by your Board. The criteria can include a petition signed by at least a specific percentage of the residents or property owners, the minimum/maximum volume of traffic that has to exist on a street to be considered, the average speed of traffic before a device will be considered, if a device may divert traffic to another street, and other parameters as may be appropriate. We believe that parameters have to be established as our past experience with traffic circles and chicanes in the Fair Oaks area, and the review of other cities' standards for installing traffic control devices, indicate that not everyone is in favor of the installation of these devices.


Vision Alignment

We believe our recommendation is consistent with Goal Number 20, "government decisions are based on careful consideration of future impact, rather than temporary relief or gain" of the Shared Vision Commitment of a "responsive, effective and collaborative government," as evaluating these two different devices will further quantify if the devices have only psychological value or can be used to control the speed of traffic commensurate with the prima facie speed limit provisions of the State of California Vehicle Code.


Fiscal Impact

The estimated cost of installing the two speed control devices and staff time involved in evaluating the effectiveness of the devices and conduct the surveys is estimated at $20,000. The study is proposed to be paid for with Road Funds. There is no impact to the General Fund.


A copy of our report has been sent to representatives of the property owners on Hillside Drive and Blenheim Avenue and we will report any comments that we receive.