Thanks to all who shared information on the potential travel impacts of
traffic calming. There is certainly plenty of good research indicating that
local street/traffic conditions affect personal travel. Studies
consistently show that pedestrian/bicycle friendly neighborhoods have
significantly (two to ten times) higher levels of walking and cycling, and
somewhat less automobile use after controlling for income and household
size. However, some of the factors involved, such as block length building
site design, are not directly affected by traffic calming.
Unfortunately, I was not able to find exactly what I wanted:
before-and-after travel studies of comprehensive traffic calming projects.
My conclusion, incorporated in our revised papers "Traffic Calming
Benefits, Costs and Equity Impacts" and "Quantifying The Benefits of
Non-Motorized Travel for Achieving TDM Objectives" (available at our
website: http://www.islandnet.com/~litman), is that comprehensive traffic
calming can typically be expected to increase walking and bicycling by
10-20%, and that half of these non-motorized trips substitute for motorized
trips. Thus, if per household non-motorized trips currently average 1.8 per
day (typical in urban neighborhoods), comprehensive traffic calming could
increase this to 2.0-2.2, and reduce 0.1-0.2 motor vehicle trips per day.
The papers discuss other factors that influence the effects of traffic
calming on vehicle travel.
I consider this a conservative estimate. The literature actually implies
that a combination of improved pedestrian/cycling conditions combined with
a reduction in vehicle speeds could have a much greater effect on travel,
both directly, and indirectly, by improving non-motorized access to public
transit, and encouraging more location efficient land use (such as more
neighborhood commercial centers).
However, most traffic calming projects appear to be rather modest in their
scope. Although they may reduce traffic speeds by 20-40% on a particular
street, they typically only cover a small portion of total streets, and a
smaller portion of total traffic mileage, in a community. For traffic
calming to have a significant effect it needs to be implemented on a
significant portion of streets in a community. This I consider to be the
limiting factor in terms of traffic calming's effects on vehicle travel.
I'd appreciate hearing any feedback on these issues, including information
on additional relevant studies, and feedback on the wording used in our
reports. Further research would be extremely useful to determine to what
degree traffic calming can be considered a TDM strategy. It would be
relatively easy if good household travel survey data were available for
neighborhoods that received comprehensive traffic calming treatments. Is
anybody doing such research now?
Sincerely,
Todd Litman, Director
Victoria Transport Policy Institute
"Efficiency - Equity - Clarity"
1250 Rudlin Street
Victoria, BC, V8V 3R7, Canada
Phone & Fax: 250-360-1560
E-mail: [log in to unmask]
Website: http://www.islandnet.com/~litman
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