following article was published in June 2002.
Caused by Traffic Calming
contribution to the local debate on the merits of speed bumps, it is worth
covering a report produced by the TRL (Transport Research Lab.) last year. In
the past, different studies in different countries seemed to produce very
diverse results, but the latest methodology seems more likely to have produced
accurate figures. TRL
Report No. 482 studies the effect of a number of different traffic calming
measures, including road humps, cushions, pinch points and mini-roundabouts.
They also studied the impact on traffic flows and delays experienced by fire
from the report “The results of the study clearly indicate that traffic calming
measures increase the emissions of some pollutants from passenger cars. For
petrol non-catalyst, petrol catalyst and diesel cars, mean emissions of CO per
vehicle-km increased by 34%, 59% and 39% respectively. For all three vehicle
categories the increase in mean HC emissions was close to 50%. Emissions of NOX
from petrol vehicles increased only slightly, but such emissions from diesel
vehicles increased by around 30%. Emissions of CO2 from each of the three
vehicle categories increased by between 20% and 26%. Emissions of particulate
matter from the diesel vehicles increased by 30%.”
The more “severe” the
traffic calming measure, for example road humps, the larger was the increase in
emissions! So without being specific, the report suggests that speed humps would
generate even worse figures than those above.
However they do say
that these increases would rarely be sufficient to cause problems and would be
unlikely to cause breaches of the Air Quality Strategy Standards, but if every
minor road in the borough had some sort of traffic calming measure, as was
likely to happen if previous policies were continued, the extra pollution would
be very substantial.
Cushions in Manor Park Rd, Chislehurst
Traffic diversion was
very variable in this study, depending on the site, but one peculiar thing they
noticed was that the mix of vehicle types after installation of the humps
changed. It appears that smaller cars diverted more with the result that larger
cars were more common as a percentage of users after installation (incidentally
this does not account for the pollution changes as it was measured in a
different way which was not affected).
This is probably
accounted for by the fact that larger cars are smoother over humps, and can
straddle cushions fully so they are not affected by the latter at all in many
cases. One peculiar result of installing speed cushions everywhere might be that
people would choose to drive bigger cars. (Editors Comment: This might explain
the recent popularity of Sport Utility Vehicles, SUVs, which are otherwise an
environmental and social disaster).
It might also explain why some
improvement in accident rates in the streets concerned is seen after
installation because it is well known that larger vehicles are much safer for
occupants when a road accident occurs - of course the overall effect is nil
because the smaller cars just have their accidents somewhere else. Along with
the general tendency to cause traffic diversion, this might help to explain why
speed humps seen to have a positive impact on accidents in the street in which
they are installed, but the overall accident figures do not fall. For example in
boroughs where speed humps are even more prevalent than Bromley, overall
accident rates have hardly improved at all in the last few years.
suggestion that installing speed cushions so as to encourage people to buy
larger cars might make sense, on the basis that if everyone had a larger vehicle
then overall road accident injuries would be cut, is an interesting idea. But
such vehicles are less efficient so there would be much higher environmental
on account of emissions directly increased and the encouragement to use larger
vehicles, speed humps and cushions are environmentally a bad idea.
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