Re-Validation of Speed / Flow Curves
Nick Woollett, AECOM, Brian Vaughan, Transdat Solutions, George Lunt, AECOM
AECOM has undertaken a review of speed/flow relationships on Highways Agency roads in England. The work has produced new relationships for light & heavy vehicles, which indicate different link speeds for vehicles than current published relationships.
Initial research of speed / flow relationships by the UK Highways Agency on three-lane motorways indicated that the observed relationship differed from that published in the Design Manual for Roads and Bridges Volume 13 (the COBA manual). The potential for speed / flow curves to evolve is plausible due to changes in driver behaviour resulting from developments in vehicle performance.
The purpose of the project was to carry out a comprehensive review of the observed speed flow relationships on the Strategic Road Network in England and, if required, produce an updated set of relationships for all road types, distinguishing between light and heavy vehicles.
The last review of the DMRB standard speed / flow relationships was undertaken by the Trasport Research Laboratory in the early 1990s. Advances in data collection technology and storage since that time provided an opportunity to quickly construct a large database of count data, average link journey times and supplementary data sources across the Highways Agency’s network for the purposes of re-validating speed / flow relationships. Analysis was also undertaken to remove, as far as possible, the effects of junctions on journey times and distinguish between light and heavy vehicles. The principal finding is that the observed speed flow relationships differ from that published in the COBA manual in that there is a third component to the observed speed flow curve for dual carriageways and motorways representing light traffic conditions where the indication is that traffic volumes have only a small impact on speeds until a certain volume of traffic is reached.
The project concluded that in general the published speed / flow relationships are not representative of average traffic conditions on HA roads, and tend to overestimate vehicle speeds on rural single carriageways and underestimate speeds on motorways and dual carriageways. This research will now be tested in traffic models to better understand the implications for traffic modelling and policy.
Association for European Transport