Induced Traffic: New Findings in the Netherlands About Induced Traffic and the Benefits of New Road Infrastructure
Han Van Der Loop, KiM Netherlands Institute for Transport Policy Analysis, Rinus Haaijer, MuConsult, Jasper Willigers, Significance
The extra traffic on the total Dutch road network induced by adding lanes on trunk roads from 2000-2012 appeared to be lower than reported in transportation literature. This and other conclusions are based on analysis of detailed data.
Increase of road capacity by adding one or more lanes existing roads, or the opening of new road(link)s, may improve the traffic flow and thereby reduce congestion. However, new road infrastructure may also attract extra traffic (‘induced traffic’) and thereby reduce the initial effect on congestion.
This paper deals with the following questions related to the phenomenon of induced traffic. How much is the increase of traffic that can be observed on the road network after the opening of new road infrastructure? How much of this new traffic on the network has been evoked by adding road capacity? What are the benefits of adding road capacity for the user?
Between 2000 and 2012 vehicle kilometres in the Netherlands increased by 16%. About 1/8 of this increase in car use (2% increase) could be attributed to traffic induced by the increase of road capacity (i.e. lane kilometres) added in the same period.
After the addition of road capacity (extra lanes), an increase in traffic of 10%-12% appeared during peak hours on roads that were congested before the addition of the extra road capacity. This increase of 10%-12% mainly consisted of car use that already existed on the network before, but used other roads and/or hours to avoid congestion. The impact of better quality on totally new car use because travellers now choose the car or travelled to further destinations was relatively small. In literature this ‘new’ car use is often referred to as ‘induced traffic’ or ‘induced demand’.
Induced traffic can be defined in different ways. The definition may include all possible behavioural reactions of travellers, but may also be limited to the extra traffic on formerly congested links during peak hours. In this paper we define induced traffic as the ‘net amount of vehicle kilometres on the total road network during working days’.
From empirical studies in transportation literature it can be concluded that the amount of traffic induced by adding 10% infrastructure (lane miles) generally is about +3% until +5%. Our analysis for the Netherlands, based on detailed traffic and other data, shows that the increase of traffic by adding 119 lanes between 2000 and 2012 on the main trunk road network is comparable (+4%) with these results. However the totally new car use on the total network is estimated to be about 2% because half of the increase of 4% was caused by cars formerly using the underlying network.
In the paper we also demonstrate that the addition of lanes provided benefits for users of infrastructure by reducing travel time delay and increasing travel time reliability.
Association for European Transport