#47 Speed Limit Compliance

The relationship between vehicle speeds, crash risk, and injury severity is well known and is a key topic that continues to be examined heavily in road safety research. Generally, both crash risk and injury severity risk increase with higher speeds. This is a fundamental driver of various efforts to reduce speeds and speeding through a combination of engineering, enforcement, education and regulatory measures. The benefits of speed reduction measures may be crudely estimated by calculating crash rates and relative impact speeds in crashes before and after an intervention. Such estimates are likely to be more reliable if they also account for levels of speed limit compliance, which vary significantly across different environmental contexts, including speed zones. Apparent compliance levels vary not only according to these factors, but also according to the data sources used.

In research published by an Australian insurer, self-reported survey data from 2023 (N=1,000) across Australia suggested that 83% of drivers normally drove at or below the posted speed limit. Isolating Queensland participants only, self-reported compliance was higher at 88%. This contrasts with other research conducted for Queensland Transport and Main Roads in 2021-22 using GPS probe data, which estimated an observed overall compliance rate of 73% across all road types[1]. Bias in the self-report survey data (e.g. socially desirable responding, participant selection) and bias in the GPS probe data likely explain some or much of this difference.

Compliance according to speed zone

Focusing on the GPS probe data for Queensland in 2022, speed limit compliance appears highest in 50km/h zones (87%), and lowest in 100km/h zones (47%). The data also indicate that of those who speed in 100km/h zones (53%), the overwhelming majority is low-level speeding (by 10km/h or less). This may help to explain why compliance is notably higher in 110km/h zones, a motorway speed with which most drivers appear satisfied.

Compliance was lower in 40km/h zones (79%) than in 50km/h zones (87%), which is concerning as the lower limit is specifically aimed at protecting vulnerable road users, including children in school zones. Moreover, of non-compliant cases in 40km/h zones, a relatively smaller proportion involved only low-level speeding. Approximately 14% of observations in 40km/h zones involved moderate or excessive speeding.

Safer speed solutions

As mentioned, a range of engineering, enforcement, education and regulatory measures are available to promote safer speeds and improved compliance. These include:

• Safe and credible limits

• Self-explaining roads

• Vehicle technologies to assist driver compliance

• Education for road users

• Effective enforcement

• Stricter legislation

[1] Kemp, A., et al. (2023). Traffic speed trends on Queensland roads, 2021-2022. Houston Kemp, Sydney.