Operating characteristics and economic evaluation of "2+1 lanes" with or without ITS assisted merging

A number of countries have adopted a three-lane configuration referred to as a "2+1 roadway" at moderate to higher traffic volumes, to bridge the gap between 2-lane and 4-lane roads. A 2+1 roadway has alternating passing lanes by direction, often utilising a continuous 3-lane cross section for some route segments. Despite its lack of New Zealand implementation, the 2+1 roadway is recognised within the NZ passing lane strategy as an intermediate step to four-laning.

Working with Peter Kirby and Bevan Wilmshurst (then at TDG), Glen Koorey (then at Canterbury University) investigated for NZTA developing, implementing and measuring safe and robust design principles and techniques to understand economic efficiency and operation on New Zealand’s highways with a focus on passing lanes and 2+1 passing facilities.

The project reviewed international experience of passing lane and 2+1 safety and operational performance, to enable the development of a 2+1 roadway design that is potentially suitable for New Zealand.

Extensive microsimulation traffic modelling was also undertaken to establish the optimum passing lane length and spacing combinations that deliver the maximum benefits for combinations of terrain, AADT, traffic growth forecasts and the percentage of heavy commercial vehicles.

In high-volume environments, the merges of passing lanes and 2+1 roadways can create delays that affect traffic flow, reduce traffic speeds and increase the risk of collisions. A range of merge concepts assisted by intelligent transport systems (ITS) were investigated to establish if the deployment of technology could enable vehicles to merge at higher volumes and thereby extend the service life of passing lanes and 2+1 roadways.