Lane distribution of traffic near merging zones influence of variable speed limits

The congestion at on-ramps of motorways is due to too many vehicles wanting to merge onto the same lane. Ramp metering is usually used as control measure to influence the flows, but a variable speed limit can also have large consequences for the merging process. This paper discusses the change in lane distribution due to a VSL and explicitly considers the influence of an on-ramp. To this end, the lane distribution just upstream of an on-ramp is compared with the lane distribution elsewhere. Just upstream of an on-ramp, a significantly lower fraction of the flow uses the outside (right) compared to a part of the road without any ramps. This holds both for a situation with VSL as without VSL. Besides, VSL increases the use of the outside lane near capacity. This way, VSL influences not only the speed but also the lane distribution, and thereby possibly also the merging ratio. The consequences of this changed lane distribution are site-dependent and should be taken into account when deciding on installing a system of variable speed limits.