Crash risk increases exponentially as average vehicle speeds increase. Speed-crash models quantify this by statistically linking percentage changes in average vehicle speeds with percentage changes in vehicle crashes and crash death and serious injury. They use real-world data collected by speed measurement devices like traffic monitoring equipment and radar.
“If the average speed travelled on a two-lane, two-way rural road decreases from 100km/h to 80km/h, for example, then deaths and serious injuries on that road will be expected to decrease by 43 percent,” says WSP research leader for road safety Bill Frith, who co-authored the paper with WSP technical group lead Fergus Tate.
Speed-crash models have become increasingly sophisticated since the first was developed in the 1980s by Swedish researcher Goran Nilsson.
When speed limits were changed on rural roads in Sweden, Goran looked at changes in actual vehicle speeds and the crashes that accompanied them. Later, the Norwegian Rune Elvik and Australian Max Cameron refined Goran’s model to be used on urban roads.
Used by transport agencies and municipalities around the world, speed-crash models play a vital role in transport safety planning. They’re no small benefit in Aotearoa New Zealand where annual road deaths consistently exceed 300.
Bill and Fergus’ paper provides guidance for anyone using speed-crash models. It recommends what type of model to use in what circumstance and encourages those who aren’t using a speed-crash model to consider doing so.
Speed-crash models now exist for all types of roads, including expressways, rural roads, urban roads, and urban arterials. Models are also available that show what happens when a vehicle hits vulnerable road users such as cyclists or pedestrians at various speeds.
The paper recommends in most circumstances that models that deal with average, or mean, changes in speed are used. Bill says these correspond to reality in all cases, except where there’s a very pronounced step-change in speed as with intelligent speed adaptation – where everyone has a device in their car that discourages or prevents the breaking of speed limits.
Because speed-crash models show that higher speeds are linked with a greater risk of road death and serious injury, Bill says it’s important that drivers ease off the pedal, stick to the speed limit, and drive to the conditions.
“It’s more important than ever that we understand the consequences when things change the speed profile of a road – such as more heavy-footed drivers, putting speed limits up or down, installing speed cameras, or changing the road from one type to another.”
To find out more about speed-crash modelling, download Bill and Fergus’ paper below.
