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Pollution Caused by Traffic Calming
   Road Safety

The following article was published in June 2002.

Pollution Caused by Traffic Calming

As a 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 engines.  

To quote 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.  


        Speed 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 other London boroughs where speed humps are even more prevalent than Bromley, overall accident rates have hardly improved at all in the last few years.

The 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 costs. 

Clearly both 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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