NZTA think a 10 km/hr increase only leads to a 2 km/hr increase

I was pleased to see the Government announce the speed limit for Transmission Gully would be consulted on increasing to 110 km/hr. New motorways are built to safety standards where this is a safe speed.

I was puzzled though as to why NZTA said that a 10 km/hr increase for a 27 km motorway would only save 18 seconds. The answer is they assume the actual average speed limit would only increase by 2 km/hr. This seems to suggest that the actual posted speed limit has little impact on how fast people drove, and they just drive to the conditions.

Also of interest is that they predict the number of serious crashes would not change, and the number of minor crashes would change from just 18 over five years to 19 over five years.