The significance of induced demand in road design: a viewpoint comparison

Abstract

In this thesis, the significance of the so-called ‘induced demand’ phenomenon is discussed. This phenomenon can occur when a stretch of road which is prone to congestion is widened. Although this widening should in theory solve congestion problems, it can often spawn additional effects, typically undesired and occasionally unforeseen. These effects could include shifts of travel in route and time (people formerly avoiding congested roads by departing at another time or taking a detour will now return to that road), but also new travels might be made which were not made before road widenings. All of these effects might eventually converge to raise traffic amounts to levels with which the widened road is just as prone to congestion as it was. This additional traffic demand which a widened or new road creates is commonly denoted as ‘induced demand’.
Although this induced demand phenomenon has commonly been acknowledged to exist, it seemed like there was little agreement on both its amount as well as its significance. Some readings seemed to suggest that for every percent of road capacity added, a percent of new traffic will occur, whereas other readings suggested that the effects were much smaller and might be limited to, e.g., merely a third of a percent extra traffic for a percent extra road capacity. A semi-systematic literature review has been carried out, enabling to compare writings of various origins, ranging from scientific papers via documents from governmental advisory bodies to newspaper articles for the general public.
These writings help to determine the significance of induced demand. It has been found that there are major discrepancies, even amongst scientific literature, concerning the ways in which the induced demand is measured, both in terms of the actual quantities which are compared to each other, as well as the time span over which the aforementioned effects are measured. These discrepancies do not aid in creating an unambiguous message for policymakers, nor do they facilitate a straightforward approach in avoiding new congestion on widened roads. Combined with the remark to be found in many scientific writings that the induced demand topic is in need of more research, a relayed recommendation from this thesis is therefore that more research be performed into this topic. However, the main recommendation is that the newly done research be more standardised, both in terms of the measured quantities and the time span over which the effects are measured.