Does speed greatly affect travel time?

Lower speeds are one of the fundamental ways to improve road safety outcomes in New Zealand and to provide more equity and access to a wider range of safe travel choices. One of the most common arguments stated against lower speed limits is the effect on travel times. While it is hard to deny the laws of physics, many people mistakenly over-estimate the impact of this change based on the relative change in speed limits (e.g. a 20% reduction in the posted speed limit is assumed to lead to a corresponding increase in travel time). Such travel time increases are blamed for loss of economic productivity and even reduced safety due to increased driver fatigue.

Actual time differences with lower speeds are generally overestimated due to the limited amount of time that one is usually able to travel at the theoretical maximum speed. Delays may arise from road geometric constraints (e.g. tight horizontal curves), other traffic (e.g. urban congestion), point restrictions (e.g. intersections, railway crossings), or section restrictions (e.g. road works, lower speed towns along a journey). In these cases, the time travelled through these sections at lower speeds will be unaffected by what the maximum limit is.

Discussion around travel time also need to consider the other "economic/efficiency" benefits that arise from introducing lower speeds. These can include greater encouragement of active modes, health and environmental benefits (incl. noise reduction), and economic gains from adjacent residents and businesses.

This presentation was given on 12 March 2020 at the Transportation Conference in Christchurch.