Designing streets for safer speeds

In the lead up to this years’ upcoming Australasian Road Safety Conference, Aini Fayaz Mansoor and Jeanette Ward give us a taste of their urban street speed prediction model research.   

There is a well-established relationship between speed and crash severity. Reducing the speed limit alone is not enough to provide effective speed management. Appropriate street design is equally important. As transport practitioners, we have the opportunity to design streets that bring about safe and appropriate speeds.

Road to zero

New Zealand’s road safety strategy, Road to Zero 2020-2030, is governed by the Safe System approach. Road to Zero 2020-2030 has a goal of reducing deaths and serious injuries on New Zealand roads by 40% by 2030. The Safe System approach aims to create a forgiving road environment. It acknowledges the fact that crashes will happen, but it is unacceptable for road users to be killed or seriously injured.

There are four guiding principles in the Safe System approach.  These are: promoting good choices but planning for mistakes; designing for human vulnerability; strengthening all parts of the road transport system and sharing the responsibility.

Street design

Street designers play a role in all four principles. Having an understanding of the impacts of street design on operating speeds is an important part of this responsibility. A robust speed prediction tool would support appropriate street design under the principles of the Safe System approach.  We conducted research to investigate how different urban street design elements affect traffic operating speed, with the aim of developing a model. The focus was on single-carriageway urban streets which have speed limits of 50 km/h or less.  Only flat streets were examined in the data analysis.

The research

The streets analysed in this study all contained a variety of road layouts, classifications, speed limits and traffic volumes. Elements of road layout included in the analysis were the presence/absence of a centreline, edge lines, medians (flush/raised), parking spaces (defined or informal), bus stops, bus lanes, cycle lanes, separated cycleways, sharrows, lane narrowings, chicanes (example below), vertical deflection devices, combination (horizontal and vertical deflection) devices, and street trees.

The findings

The research found that there are aspects of street design that influence operating speeds with varying effectiveness, depending on the combinations of design features. However, more research is required to establish robust relationships between them to develop a useful mathematical model. The impact of human behaviour on operating speeds appears to be significant. Further research is required to consider how driver behaviour and urban street design interacts to understand the relationship between them, prior to developing the model.

Design guidance should embed clear, directive advice on design aspects that contribute to appropriate operating speeds. To hear more about this fascinating research, join us at the Australasian Road Safety Conference on 28-30 September where Jeanette Ward and Aini Fayaz Mansoor will present their findings.

street design