Modelling an Optimal Speed Limit

Recent Government policy introduced new Setting Speed Limit Rules aiming to increase or reverse many lowered speed limits; the rationale being to “help drive economic growth and improved productivity”. However, recent policy modelling would suggest that, in many cases, a lower speed limit will be the more “optimal” one. 

An economic assessment of NZ State Highway speed limits, taking into consideration crash, travel-time, air pollution, and vehicle operating costs, found that optimum limits for most undivided highways were <80 km/h (less again for heavy vehicles). Even on motorways, the optimal speed limit was <100 km/h for light vehicles. Notably for heavy commercial vehicles, they are limited to a maximum legal speed of 90 km/h, and their size and aerodynamic impact mean that their fuel efficiency “sweet spot” speed is much lower than light vehicles.

Separately, ViaStrada was commissioned to assess the economic benefits of four urban and four rural options for the proposed Nelson/Tasman Regional Speed Management Plan. An assessment was undertaken using standard NZTA MBCM parameters of the likely benefits and dis-benefits related to lowered travel speeds on some roads. For each road section in the network, an estimate of the likely changes in speeds from the existing recorded mean speeds was determined.

Three of the four rural speed management options produced strong positive BCRs (5:1 to 9:1), largely due to good expected crash reductions and reduced VOCs and emissions. For urban areas, the benefits were less clear (partly due to modelling limitations); however, there are likely to be other benefits there that are more difficult to accurately quantify, including health benefits from active mode shift, improved amenity, and reductions in air pollution and noise. 

This presentation summarises these investigations and offers some considerations for RCAs reviewing their own speed management strategies.