A grand bargain

Cleveland Street at City Road crash (across the street from the discussed island, adjacent to another refugee island, for free lefts from City Road to Cleveland Street). Fortunately there was guardrail, I guess. Speed limit, 50 km/h. The police officer was not pleased by the photographers documenting the failure of traffic engineering.

We know that lowering vehicle speeds on residential streets increases safety, reduces traffic, and that low traffic neighbourhoods encourage more people to walk and bike.

We know that raising vehicles speeds on motorways will attract drivers from local roads to motorways, may encourage more travel by motorway (and maybe overall), but will increase safety risk on motorways (but arguably might reduce risk overall, as drivers divert from less safe roads to safe roads).

Doing both could be a politically powerful bargain, improving our urban environment. But how does this net out? Will the safety improvements on local streets from fewer vehicles and lower speeds outweigh the increased risk on motorways?

Because we don’t know how this nets out, and the risks of being wrong are fatal, I propose that we study this. 

  • Hypothesis: Raising speed limits on motorways and lowering speed limits on local roads in urban areas reduces traffic deaths per capita.

How do we test this?

We can simulate this in silica, but that only goes so far. We don’t really know the demand response to complex changes in travel speed limits (how much does speed actually change when the speed limit changes depends on the design of the road), and models are not particularly accurate. In the end, we will need to do trials, find a motorway network (or multiple networks) and set of local roads where the change can be implemented, along with a control case where it is not, and compare.

The evaluation should assess traffic safety, mode shares, and environmental impacts, along with speeds and speed limit compliance.

The answer could make it politically expedient to lower urban speed limits, and shift car crash risk from pedestrians and bicyclists to motorists. I look forward to seeing the answer.

Published by David M Levinson

Prof. David Levinson teaches at the School of Civil Engineering at the University of Sydney, where he leads TransportLab and the Transport Engineering group.

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