Do 20mph zones have a negative impact on the environment?

[Todd Litman]

There are a number of vehicle operating cost models which include fuel
consumption/speed curves for highway travel. These include:

Peter Bein, et al., British Columbia Vehicle Operating Costs, Planning
Services Branch, British Columbia Ministry of Transportation and Highways
(Victoria, www.th.gov.bc.ca/bchighways), 1996.
Highways Design and Maintenance (HDM) 4 Model, World Bank (Washington DC;
www.RoadSource.com).
MicroBENCOST, Texas Transportation Institute (http://tti.tamu.edu), 1997.

The ones I've seen indicate that automobile fuel consumption is nearly flat
between about 20 and 50 mph. There may be a slight reduction when you go
from 20 mph to 30 mph, but it is just a few percent. Only below about 15
mph do you start seeing a significant increase in consumption rates. These
curves represent unimpeded highway travel, I expect that minimal fuel
consumption speeds would be somewhat lower under urban stop-and-go travel
conditions, reducing maximum speeds reduces acceleration, which consumes
lots of energy. My conclustion then is that the fuel consumption rates per
mile would probably stay about the same if average traffic speeds declined
from about 30 mph to about 20 mph.

Reducing speeds is also likely to reduce total vehicle travel, resulting in
additional fuel savings, which I suspect would more than offset any slight
increase in fuel consumption per mile. Many studies indicate that vehicle
mileage has an elasticity of  -0.5 or more with respect to travel time,
meaning that a 1% increase in travel time should reduce total vehicle
travel by 0.5% or more. Reducing average travel speeds from 30 mph to 20
mph increases travel time by 33%, which should reduce total vehicle travel
by 16% or more.

Vehicle use imposes a number of other environmental impacts besides CO2
emissions. Emission rates vary from one to another, but most decline with
vehicle speeds. For these reasons, I would not agree with the claim that a
20 mph speed limit has a "negative effect on the environment." At most,
some environmental impacts could increase slightly.


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:  litman at vtpi.org
Website: http://www.vtpi.org



At 01:28 PM 03/08/2000 -0000, lucy.knight at ic.ac.uk wrote:
>The UK Road Safety Strategy recently launched by the government stated that
>an urban 20 mph speed limit would increase CO2 emissions and have a negative
>effect on the environment.
>
>In the next paragraph they gave support to self-enforcing 20mph zones in
>urban areas especially around schools.  If anything I would think
>self-enforcing 20mph zones where vehicles are forced to slow down and speed
>up would not improve the quality of life for the immediate population, due
>to noise and vehicle emissions.
>
>However, it made me wonder if a balnket speed of 20mph would be detrimental
>to the environment, does anybody know the answer or have any commnets about
>20mph zones?  

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:  litman at vtpi.org
Website: http://www.vtpi.org