Bicycle Safety Guide and Countermeasure Selection System
This diverter in San Luis Obispo, CA, is one component of a bicycle boulevard (also pictured below). http://www.pedbikeimages.org/ - Adam Fukushima
Traffic diversion techniques are remedies intended primarily to reduce traffic volumes on residential neighborhood streets when traffic calming or other measures have not sufficiently reduced cut-through traffic. The prime beneficiaries of traffic diversion are bicyclists, pedestrians, and those who live on the treated streets, but local residents are also most negatively affected by traffic diversion.
A diverter is an island built at a residential street intersection that prevents certain through and/or turning movements. There are four types of diverters:
Other types of island diverters can be placed on one or more approach legs to prevent through and left-turn movements and force vehicles to turn right (see turning restrictions). Diverters may also be used in conjunction with other measures to create "bicycle boulevards," specialized streets that give priority to through movement of bicyclists, but at intervals divert motorized traffic in order to provide a preferential bicycling environment. Examples of bicycle boulevards may be found in Palo Alto, CA (see case study).
Traffic diversion should only be used as a last resort, and then only in conjunction with area-wide traffic analyses and management. Any of the diverters listed above can be designed for bicycle and pedestrian access.
Street Closures A partial street closure uses a semi-diverter to physically block one direction of motor vehicle travel into or out of an intersection; it could also involve blocking one direction of a two-way street. Partial street closures at the entrance to a neighborhood or area should consider the traffic flow pattern of the surrounding streets as well. A partial closure provides better emergency access than a full closure. Since this design also allows motorists to easily violate the prohibition, police enforcement may be required. If the partial closure only eliminates an entrance to a street, a turnaround is not needed; closing an exit will generally require a turnaround.
A full street closure is accomplished by installing a physical barrier that blocks a street to motor vehicle traffic and provides some means for vehicles to turn around. There are a number of considerations before implementing a full street closure, which should be used only in the rarest of circumstances. Neighborhoods with cul-de-sac streets require extensive out-of-the-way travel, which is not a mere convenience issue, but has serious implications for impacts on other streets. All traffic is forced to travel on feeder streets, which has negative consequences for the people who live on those streets and forces higher levels of control at critical intersections. Provision for emergency vehicle access should also be made. Such provision can be accomplished with a type of barrier or gate that is electronically operated, or by installing barriers that permit only large or wide-axle vehicles to traverse them.
Traffic diversion augments traffic calming by limiting motor vehicle traffic on certain streets and preventing turns from an arterial street onto a residential street. It also reduces traffic volume by discouraging or preventing traffic from cutting through a neighborhood and restricts access to a street without creating one-way streets.
Diverters cost in the range of $15,000 to $45,000 each, depending on the type of diverter and the need to address drainage. The costs presented in the table below are limited to full diverters and truncated diagonal, or semi-, diverters. The cost of installations will vary based on the amount of material needed and the drainage needs at the site.
