NETWORK MODELLING OF THE TRAFFIC, ENVIRONMENTAL AND ENERGY EFFECTS OF LOWER URBAN SPEED LIMITS.

Recent studies have indicated that lower urban speed limits offer significant road safety benefits. An issue which has arisen in assessing the importance of these benefits is the likely impact of lower speed limits on other aspects of road travel, such as mobility and travel time, fuel consumption and emissions. This paper examines preliminary indications of the possible effects of reduced speed limits on these factors, using the TrafikPlan traffic network analysis model. It considers the effects of lower speed limits and speed zoning, as applied to a range of urban road and street types, on journey times, mobility and accessibility, and fuel consumption and emissions in urban and suburban areas. The paper outlines the results found for a synthetic grid network to which two different traffic design standards were applied. The network was modelled at four different levels of traffic congestion, ranging from light to moderate traffic, to oversaturated conditions. For each combination of network and traffic congestion level, three speed limit regimes (60, 50 and 40 km/h) were applied and modelled using TrafikPlan. The study concludes that: (1) journey speeds in the test networks were considerably less than the set speed limits; (2) differences in overall travel speeds and journey times were much less than the differences in the speed limits themselves; (3) signal coordination offered significant advantages for delays and traffic progression, except at higher congestion levels where some oversaturation was experienced; and (4) an argument for public acceptance of lower speed limits could be based on an improvement in traffic progression and quality of traffic flow possible under the lower speed limit regimes.