Induced demand, or latent demand, is the phenomenon that after supply increases, more of a good is consumed. This is entirely consistent with the economic theory of supply and demand; however, this idea has become important in the debate over the expansion of transportation systems, and is often used as an argument against widening roads, such as major commuter roads. It is considered by some to be a contributing factor to urban sprawl.
Effect in transportation systems
Latent demand has been recognised by road traffic professionals for many decades. J. J. Leeming, a British road-traffic engineer and county surveyor between 1924 and 1964, described the phenomenon is his 1969 book:
Motorways and bypasses generate traffic, that is, produce extra traffic, partly by inducing people to travel who would not otherwise have done so by making the new route more convenient than the old, partly by people who go out of their direct route to enjoy the greater convenience of the new road, and partly by people who use the towns bypassed because they are more convenient for shopping and visits when through traffic has been removed.
He went on to give an example of the observed effect following the opening of the Doncaster Bypass section of the A1(M) in 1961.
Price of road travel
A journey on a road can be considered as having an associated cost or price (the generalised cost, g) which includes the out-of-pocket cost (e.g. fuel costs and tolls) and the opportunity cost of the time spent travelling, which is usually calculated as the product of travel time and the value of travellers' time.
When road capacity is increased, initially there is more road space per vehicle travelling than there was before, so congestion is reduced, and therefore the time spent travelling is reduced – reducing the generalised cost of every journey (by affecting the second "cost" mentioned in the previous paragraph). In fact, this is one of the key justifications for construction of new road capacity (the reduction in journey times).
A change in the cost (or price) of travel results in a change in the quantity consumed. This can be explained using the simple supply and demand theory, illustrated below.
Elasticity of transport demand
For roads or highways, the supply relates to capacity and the quantity consumed refers to vehicle-kilometres travelled. The size of the increase in quantity consumed depends on the elasticity of demand.
A review of transport research suggests that the elasticity of traffic demand with respect to travel time is around −0.5 in the short-term and −1.0 in the long-term. This indicates that a 1.0% saving in travel time will generate an additional 0.5% increase in traffic within the first year. In the longer-term, a 1.0% saving in travel time will result in a 1.0% increase in traffic volume.
Sources of induced traffic
In the short term, increased travel on new road space can come from one of two sources: diverted travel and induced traffic. Diverted travel occurs when people divert their trip from another road (change in route) or retime their travel (change in timing). For example, people might travel to work earlier than they would otherwise like, in order to avoid peak period congestion – but if road capacity is expanded, peak congestion is lower and they can travel at the time they prefer.
Induced traffic occurs when new automobile trips are generated. This can occur when people choose to travel by car instead of public transport, or decide to travel when they otherwise would not have.
Shortening travel times can also encourage longer trips as reduced travel costs encourage people to choose farther destinations. Although this may not increase the number of trips, it increases vehicle-kilometres travelled. In the long term, this effect alters land use patterns as people choose homes and workplace locations farther away than they would have without the expanded road capacity. These development patterns encourage automobile dependency which contributes to the high long-term demand elasticities of road expansion.
Induced demand and transport planning
Although planners take into account future traffic growth when planning new roads (this often being an apparently reasonable justification for new roads in itself – that traffic growth will mean more road capacity is required), this traffic growth is calculated from increases in car ownership and economic activity, and does not take into account traffic induced by the presence of the new road (i.e. it is assumed that traffic will grow, regardless of whether a road is built or not).
In the UK, the idea of induced traffic was used as a grounds for protests against government policy of road construction in the 1970s, 1980s and early 1990s, until it became accepted as a given by the government as a result of their own SACTRA (Standing Advisory Committee on Trunk Road Assessment) study of 1994 . However, despite the concept of induced traffic now being accepted, it is not always considered.
A comparison of congestion data from 1982 to 2011 by the Texas A&M Transportation Institute clearly demonstrated that additional roadways reduced the rate of congestion increase. When increases in road capacity were matched to the increase demand, growth in congestion was found to be much lower.
Reduced demand (the inverse effect)
Just as increasing road capacity reduces the cost of travel and thus increases demand, the reverse is also true – decreasing road capacity increases the cost of travel, so demand is reduced. This observation, for which there is much empirical evidence, has been called Disappearing Traffic, also traffic evaporation or traffic suppression. So the closure of a road or reduction in its capacity (e.g. reducing the number of available lanes) will result in the adjustment of traveller behaviour to compensate – for example, people might stop making particular trips, condense multiple trips into one, retime their trips to a less congested time, or switch to public transport, walking or bicycling, depending upon the values of those trips or of the schedule delay they experience.
Reduced demand has been demonstrated in a number of studies associated with bridge closings (to be repaired) or major roads rehabilitation projects. These studies have demonstrated that the total volume of traffic, considering the road or bridge closed and alternative roads which this traffic is diverted through, is lower than that in the previous situation. In fact, this is an argument to convert roads previously open to vehicle traffic into pedestrian areas, with a positive impact on the environment and congestion, as in the example of the central area of Florence, Italy.
Similarly, reducing public transit services will reduce to some extent the use of those facilities, where trips again may be abandoned or switched to private transport.
- Downs–Thomson paradox
- Lewis–Mogridge Position
- Say's law
- Positive feedback
- Traffic flow
- Schedule delay
- J. J. Leeming (1969). Road Accidents: Prevent or Punish. Cassell. ISBN 0304932132.
- Goodwin, P. B. (1996). "Empirical evidence on induced traffic: A review and synthesis". Transportation 23: 35–54. doi:10.1007/BF00166218.
- Litman, T. L. (2011). "Generated Traffic and Induced Travel: Implications for Transport Planning" (PDF).
- Texas A&M Transportation Institute. "2012 Urban Mobility Report". Retrieved May 14, 2013.
- Giles Duranton, Matthew A. Turner (2010), The Fundamental Law of Road Congestion: Evidence from US cities, University of Toronto
- UK Department for Transport guidance on modelling induced demand
- A statistical analysis of induced travel effects in the US mid-Atlantic region (Fulton et al.), Journal of Transportation and Statistics, April 2004 (PDF)
- Todd Litman (2001), “Generated Traffic; Implications for Transport Planning,” ITE Journal, Vol. 71, No. 4, Institute of Transportation Engineers (www.ite.org), April, 2001, pp. 38–47.