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. Traffic diversion should only be used as a last resort, and then only in conjunction with area-wide traffic analyses and management. 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.
Raised, island diverters may be used for area-wide traffic management. Four types of island diverters are diagonal, star, forced turn and truncated. A diagonal diverter breaks up cut-through movements and forces right or left turns in certain directions. A star diverter consists of a star-shaped island placed at the intersection, which forces right turns from each approach. A truncated diagonal diverter is a diverter with one end open to allow turning movements. 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. Neighborhoods with a grid-type pattern may benefit most from use of one or more of these types of diverters to reduce the appeal of neighborhood streets to cut-through traffic.
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. Local access for motor vehicles is
maintained, but traffic calming and traffic control devices help to keep
motorized speeds low and reduce conflicts between motor vehicles and bicycles.
Examples of bicycle boulevards may be found in Palo Alto, CA (see case
A partial street closure uses a semi-diverter to physically close or 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. The design of this measure should allow for easy access by bicyclists and all pedestrians. 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. If a street closure is implemented, it should always allow for the free through movement of all pedestrians including wheelchair users, and bicyclists. 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-axled vehicles to traverse them.
- Limit motor vehicle traffic on certain streets.
- Prevent turns from an arterial street onto a residential street.
- Reduce traffic volume by discouraging or preventing traffic from cutting through a neighborhood.
- Restrict access to a street without creating one-way streets.
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- Part of an overall traffic management strategy.
- Design diverters to allow bicycle, pedestrian, and emergency vehicle access. If this cannot be done and the street is a major bicycle corridor, a diverter should not be used.
- At full closures, provide a turnaround area for motor vehicles, including service vehicles, and provide for surface drainage.
- Full street closures may be considered for local streets, but are not appropriate for collector streets.
- Consider whether less restrictive measures would work. Local residents will be most affected.
- Assess whether other local streets would receive diverted traffic and/or access into or out of the neighborhood would be adequate.
- The impact on school bus routes and service vehicles should also be considered.
- Diverters generally do not effectively address midblock speeding problems1; use in conjunction with traffic calming measures if speeding is a problem.
- Diagonal diverters may be used in conjunction with other traffic management tools and are most effective when applied to the entire neighborhood street network.
- Partial or full street closures and area-wide use of diverters should have strong neighborhood support. There may be legal issues.
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The cost for a full, landscaped street closure varies from approximately $30,000 to $100,000, depending on conditions.
A well-designed, landscaped partial street closure at an intersection typically costs approximately $10,000 to $25,000. They can be installed for less if there are no major drainage issues and landscaping is minimal.
Diverters cost in the range of $15,000 to $45,000 each, depending on the type of diverter and the need to address drainage.
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