Safe speeds – what is safe enough?

How can we reduce travel time? How do we solve congestions in our cities? What are the travel needs of commuters? How can we keep our children safe as they leave school? These are a few of the important questions that influence approaches to prescribing speed limits. This has resulted in inconsistencies in setting speed limits across various geographies, leading to a patchwork of approaches. These apparent variations in the experiences of the public breeds mistrust and is a real barrier to improving the safety of our roads.

Setting and enforcing safe speed limits is ultimately a life-saving approach that places the protection of the road user at the heart of mobility needs. In ideal circumstances there is a sweet spot, promoting both mobility and safety. At the heart of current best practices is a philosophy described as the Safe System approach. This provides guidance that supports the setting of safe traffic speeds that seek to protect all road users from harm.

The Safe System approach states that no aspect of the safety of the road system should be compromised for greater mobility [3]. With this basis at its core, speed limits (and therefore maximum traffic speeds) are set through a Safe System approach in the context of the possible collision types that can occur on a road section. The logic behind setting safe speeds is rooted in physics. Vehicle speeds strongly influence the likelihood of a crash occurring and, in the event of a crash, the severity of the injuries suffered by the victims.

The complexities of the interplay between speed, journey time and risk therefore demands a scientific, evidence-based and consistent approach that considers the unique characteristics of each road section.

TRL has produced a Speed Management and Assessment Tool (SMAT) which systematically manages speed across networks and on specific stretches [4] of roads. It is a risk-based approach which determines the appropriateness of existing speed limits by assessing the level of safety provided by the road characteristics present and how these impact the likelihood of various potential collision types. It requires data on current traffic speeds, speed limits, traffic volume, road characteristics, number of lanes, driveways and traffic signals. The tool takes iRAP coding data and repurposes it to help stakeholders in a robust intelligence-led approach to defining Safe System speed limits without the need for additional data collection. The SMAT can also be used to understand where traffic speeds are in conflict with these limits.

It supports road authorities and key stakeholders by providing simple but powerful information to support with understanding what the recommended speed limits on road sections should be. It can help inform decision-makers with making evidence-based decisions to manage vehicle speeds and, ultimately, improve road safety. With the kind permission of National Highways, who supported the development of this tool, TRL has successfully applied the tool in Low- and Middle-Income Countries (LMICs), such as Ghana and Nepal, where speed management is critical, yet access to data is limited.

Setting appropriate speed limits with consideration to the different levels of vulnerability and road resilience provide a clear pathway towards reducing or eliminating the severe outcomes of road crashes. It is imperative for all professional, decision makers and road users to work towards safer speeds in a harmonised and evidence-led manner.