Images: Highways England
MODELLING WORLD 2022 SHOWGUIDE
No one should doubt the existence of generated traffic, and the need to reflect it in our models. But how?
Modelling an emotive topic – dealing with induced demand It’s one of those topics that doesn’t want to go away – whether roads generate traffic and if transport modelling reflects the induced demand phenomenon well enough. Tom van Vuren considers the evidence I was inspired to revisit the topic when I was pointed towards a paper that said “Ignoring induced demand is engineering malpractice”,* and there’s a lot in that document that I strongly disagree with. But it’s a good example of how emotive and, in my view, often ill-informed the discussion on the topic remains. What’s in a name? Some talk about induced demand, others about induced traffic, and the original SACTRA Report “Trunk Roads and the Generation of Traffic” used the term generated traffic, and that’s the term I’m settling on. Does it matter? I would say yes. As so often is the case, terms that sound similar at first sight can mean something quite different to different people. Generated traffic has been described slightly differently by many authors, but to me and others it reflects that investing in roads, increasing capacity and removing bottlenecks has been found to result in
4
more car-based traffic on the road itself and/or in the overall study area, in the time period considered, negating some or most of the intended benefits. Where that demand comes from has been less concerning for many, particularly if the objective of the road scheme had been congestion reduction – the ‘what’ being more interesting than the ‘why’. But for me, as a transport planner, it does matter where that new, generated traffic comes from. Did it previously use other modes, perhaps reducing the commercial base for public transport as an alternative? Have drivers changed destination, lengthening their distance travelled, but also potentially contributing to decongestion benefits elsewhere, outside of our immediate area of interest? Did they return to the peak? Or are these completely new trips, true induced travel demand? Thinking along these lines, studies have
shown that much generated traffic is actually redistributed from other alternatives (modes, destinations and time periods) that we already include as explicit choices in most of our (strategic) transport models. The Amsterdam Ring Road was an early case study in 1996, where an additional river crossing generated what was estimated to be 5% extra traffic after one year, and about 7% after five years. A detailed analysis of the generated traffic suggested that some destination changes had taken place, a substantial amount of trip re-timing, hardly any mode shift and a surprising amount of changes in car occupancy. A later 2013 Department for Transport (DfT) study of completing the Manchester Motorway Box estimated that generated effects consisted mainly of destination changes (and that these were strongest for non-commute purposes), with some mode shift. In the Manchester case hardly any time period
