How much do drivers actually slow down for lower speed limits?

Does reducing speed limits actually work?


Whether it is lower limits around schools or shopping areas, on rural roads or state highways, 11,000 kilometres of roads have had speed limits reduced in the last few years.

On current trends, about 30,000 kilometres of the 94,000 kilometres of all roads in New Zealand will have lower speed limits by the end of the decade, with many councils currently working on speed changes.

As these are the biggest changes in speed limits New Zealand has seen for a long time, the AA Research Foundation wanted to understand how drivers were adapting to these changes.

Researchers from Waikato University were commissioned to analyse real-world before-and-after speeds on eight state highways that had limit reductions in 2020 or 2021. The project also involved simulator testing of people’s driving on higher-speed and urban roads that had had limits lowered recently.

80km/h speed sign.

“Lower speed limits are the main tool being used to try and achieve the Government’s targets for improving road safety in New Zealand. It’s well proven that even a small reduction in speed across large numbers of vehicles does see less harm from crashes,” says AA Chief Policy and Advocacy Officer, Simon Douglas.

“But one of the things we wanted to explore in this project was seeing how travel speeds actually changed following limits being reduced, if that changed over time and if results varied at different locations.”

Speed data from 30 sites on eight highways was analysed just before the lower limit was introduced, then at regular intervals for up to seven weeks after. Most stretches of road had gone from a 100km/h limit to 80km/h. 

At 22 of the 30 locations, the average speeds of vehicles dropped almost immediately upon the limit changing and remained that way seven weeks on. Some locations had small drops in average speeds; others had larger reductions (ranging from 1km/h to 8km/h)

However, eight of the 30 locations did not see speeds reduce. Two had no change and six had an increase – i.e. vehicles were driving faster on average seven weeks after the limit was reduced.

The table below shows results across the 30 locations the researchers monitored:

Average speeds seven weeks after the limit was reduced  
At or below 80km/h 33% of locations
81-83km/h 27% of locations
84-86km/h 23% of locations
87km/h+ 17% of locations

The researchers found that the physical features of the roads, such as how wide they were could explain some of the variations in speeds at different locations.

Professor Sam Charlton, who led the work, believes drivers’ habits on different types of roads are as important as the physical features of roads in influencing travel speeds.

“We saw speeds come down at many sites and it happened within a week from the limit lowering in many cases.

“That being said, even with the most generous of criteria for success, there were certain sites that did not see average speeds come down and at all locations there were still a significant number of drivers travelling well above the new lower limits.

“When we drive a road all the time it can be hard to break old habits and we think this is responsible for much of what we saw in the data.”

New Zealand road with 80km/h sign.

From the AA’s perspective, the research appears to reinforce the view that more needs to be done than simply changing the signs on the side of the road to make limit changes effective.

While the limit was consistently 80km/h, there were substantial variations in speeds being travelled on different highways, and even at different locations on the same highway.

“Decisions around speed limits need to take into account the physical road environment and the habits drivers currently have, in terms of how fast they are used to travelling,” says Simon.

“Some roads are going to be a better natural fit for a lower limit, but on wider, straighter sections of highway, not so much.

“We also think the authorities could be doing a lot more to recognise that they are trying to change habits built over many decades around subconscious speed choices. They need to use different approaches with signage and road markings to keep reminding people what the limit is.”

Read the more about the AA Research Foundation project here. 


Story by Dylan Thomsen for the Spring 2023 issue of AA Directions Magazine. Dylan Thomsen is the Safety, Communications and Research Manager for the AA's Motoring Policy division.

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