Lower urban speed limits: what are the pieces of the jigsaw telling us at this point in time?

At present, lower urban speed limits (LUSL) have been applied to residential streets in many jurisdictions in Australia. What is the emerging evidence to indicate if they are effective in terms of various measures ? The purpose of LUSL is to reduce speeds, thereby enhancing road safety and improving the amenity value of local streets for residents. What is the magnitude of any benefits the implementations are achieving - do we yet have enough of the pieces of the jigsaw to see what the picture is ? What do we mean if we say that the 40km/h limit is working? Do we mean something materially different if we say that the 50 km/h limit is working ? Although commonly justified on the grounds of road safety, which is readily measurable for a large scheme, the application of LUSL reaches well beyond the call for improved road safety statistics, and an assembly of less well defined factors is involved. Indeed, the support for such schemes could be seen as a cry from the community for some concept of improved amenity for which traffic speed is just an inverse proxy. This consideration is reflected in other approaches in Europe where a framework for speed limits is evaluated in a more holistic light. This paper seeks to present evidence quantifying the impacts of LUSL in terms of measured speeds and volumes, community attitudes, environmental impacts, travel times and road safety outcomes based on published and emerging evidence. Much of the evidence is based on research into the city wide Unley 40km/h scheme in Adelaide and computer simulation modeling of the mobility and environmental effects of LUSL. (a) For the covering entry of this conference, please see IRRD abstract no. E202589.