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Date: Mon, 07 Jun 1999 07:10:22 -0700
From: Todd Litman <[log in to unmask]>
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Subject: Re: TLCNet: Bikeways and Impervious surfaces.
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Posted by Todd Litman <[log in to unmask]>
At 10:26 AM 6/6/99 -0400, Robert B. Bennett wrote:
>There is a plan afoot in my area (Newark, Delaware) to construct a new
>bikeway, which for all its positive features, raises the spectre of more
>impervious surfacing in our local watersheds that are already stressed.
>Also, there has been a lot of so-called "Greenways" construction elsewhere
>in our general area, that has blacktopped a lot of open space. Has anyone
>addressed this problem elsewhere and does anyone know of a bikable
>surfacing that is made of pervious material?

I think that it is good to raise the issue because paving land for roads
and parking facilities has significant environmental impacts that should be
considered in transport planning, but I don't think that paved trails cause
nearly as much problem because they are narrow and because of their use.
Our report "Land Use Impact Costs of Transportation" describes these
impacts and "Pavement Busters Guide" discusses how to reduce them (both are
available free at our website: For more
detailed information on motor vehicle water quality impact see our report
"Transportation Cost Analysis".

Let's compare the differences between parking/roads and trails with respect
to these impacts:

* Reduced stormwater recharge.
When large areas of land are paved it becomes necessary to channel
stormwater into drains in order to avoid flooding. Trails are narrow and
stormwater is usually allowed to drain off onto the side where it soaks
into the soil.

* Reduced water quality.
Stormwater from roads tends to be contaminated by dripping vehicle fluids
(crankcase oil, gasoline, coolant, etc.) and particulates (tire and brake
lining wear, road dust, etc) which can degrade surface and groundwater
(think of the oily sheen that usually develops on roadway puddles).
Non-motorized travel does not produce such harmful waste. You may have the
odd dropped ice cream cone or leaking water bottles, but impacts on water
quality should be minimal.

* "Heat Island" effect.
Dark pavement, such as asphalt, gains heat under the sun, resulting in
excessive temperatures in areas that have lots of pavement. This is both
uncomfortable and increases cooling costs during the summer in many cities.
Trails are often made of concrete, which causes much less heat gain, and
they are frequently located in areas that are shaded by trees.

* Loss of greenspace.
Of course, any type of human development tends to displace existing
greenspace, but there would be no difference between a paved and an unpaved
trail surface.

* Encourages low density urban expansion ("sprawl").
Roads, particularly increased highway capacity and parking facilities, tend
to encourage urban sprawl by degrading the urban environment (Who wants to
live next to heavy traffic?) and by accommodating longer vehicle trips.
Trails don't tend to do this because non-motorized transport has minimal
pollution or accident risk impacts and there is a natural limit to walking
and bicycling distances.

My conclusion is that most of the concerns raised about increased roads and
parking facilities do not apply very much to trails, and that negative
impacts are generally more than offset by the reducions in motor vehicle
use that can result. For more information see our report "Quantifying the
Benefits of Non-Motorized Travel
for Achieving TDM Objectives," also available at our website.

There ARE alternative surfaces that may be appropriate for relatively
light-use recreational trails, but I would not recommend them for heavy-use
trails that serve urban transportation purposes since they are unsuitable
for narrow tire bicycles. These include gravel, cinder, and a type of
pavement blocks that has gaps for soil that cover about half of the
surface. There is a picture of these blocks in "Pavement Busters Guide".
They are ideal for light-duty parking surfaces and a few communities
encourage this use in zoning codes. I don't think that they are used nearly
as frequently as they should be. However, they produce a "washboard"
surface that would be uncomfortable for bicycling.

I think that you could achieve the most environmental benefit by paving
trails for bicycling and encouraging a reduction in pavement for parking
and roads, and the use of alternative pavement surfaces that are not
entirely impervious to water for light use parking facilities and alleys.

For more information on impervious surface impacts you may want to visit
the NEMO Project ( and the Center for Watershed
Protection ( Other sources of information are
listed at the end of our papers.


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]

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