Relationships between highway capacity and …

1 Relationships between highway capacity and induced vehicletravelRobert B. Noland*University of London Centre for Transport Studies, Department of Civil and Environmental Engineering,Imperial College of Science, Technology and Medicine, London SW7 2BU, UKReceived 4 March 1999; received in revised form 1 July 1999; accepted 6 July 1999 AbstractThe theory of induced travel demand asserts that increases in highway capacity will induce additionalgrowth in tra c. This can occur through a variety of behavioral mechanisms including mode shifts, routeshifts, redistribution of trips, generation of new trips, and long run land use changes that create new tripsand longer trips.

2 The objective of this paper is to statistically test whether this e ect exists and to empiricallyderive elasticity Relationships between lane miles of road capacity and vehicle miles of travel (VMT). Ananalysis of US data on lane mileage and VMT by state is conducted. The data are disaggregated by roadtype (interstates, arterials, and collectors) as well as by urban and rural classi cations. Various econometricspeci cations are tested using a xed e ect cross-sectional time series model and a set of equations by roadtype (using Zellner s seemingly unrelated regression).

3 Lane miles are found to generally have a statisticallysigni cant relationship with VMT of about in the short run and between and in the long are larger for models with more speci c road types. A distributed lag model suggests a rea-sonable long-term lag structure. About 25% of VMT growth is estimated to be due to lane mile additionsassuming historical rates of growth in road capacity . The results strongly support the hypothesis that addedlane mileage can induce signi cant additional travel. 2000 Elsevier Science Ltd.

4 All rights IntroductionThe theory of induced growth in vehicle travel hypothesizes that increases in the carrying ca-pacity of a speci c highway corridor or road network will attract increased levels of vehicle tra economic interpretation of travel demand would argue that cost does in uence demand fortravel. These costs include both the capital costs of a vehicle plus fuel and maintenance costs, asTransportation Research Part A 35 (2001) 47 *Tel.: +44 (020) 75946036; fax: +44 (020) ( Noland).0965-8564/00/$ - see front matter 2000 Elsevier Science Ltd.

5 All rights : S 0 9 6 5 - 8 5 6 4 ( 9 9 ) 0 0 0 4 7 - 6well as the relative travel time costs within a given network. Increases in highway capacity shouldreduce the cost of travel if relative travel times are reduced, resulting in an overall increase indemand. This phenomenon of induced demand due to capacity expansions is analyzed in an economic perspective this would be a trivial argument. However, the demand fortransportation has historically been characterized as a derived demand; , households onlydemand transportation in the course of carrying out other economic activities and not for thepleasure of movement in and of itself.

6 From this assertion it follows that household demand forvehicle travel is only determined by demand for exogenous economic activities, and the cost oftravel is considered virtually irrelevant. While this is obviously an extreme interpretation, theconsideration of travel as a consumed economic commodity has not in uenced overall UStransportation theory of induced growth in vehicle travel has been periodically cited in the literature formany years. Goodwin (1996) cites at least one report dating back to 1938 that documented evi-dence for this e ect.

7 Since then the discussion of induced vehicle travel has been periodicallydebated and generally been discounted or considered a minor e ect by policy makers. The recentSACTRA (1994) report in the UK changed much of this debate and was an acknowledgement bythe UK government that many road projects generate extra tra the US the debate on induced travel remains controversial. The debate is generally betweenthe environmental community and traditional transportation decision makers. Much of the e ortand debate have focused on the modeling procedures used in regional travel demand forecasting(Coombe, 1996; Mackie, 1996).

8 These models are recognized as generally not being able to ac-count for various induced travel e ects. Minor upgrades to current modeling practice, such asrecalibrating trip distribution models based on changes in travel speeds and inclusion of modechoice and route choice procedures can account for some of the increases in vehicle miles of travel(VMT). However, these modi cations would not measure changes in trip generation (US DOT,1996) or any impact of long run land use changes. Activity-based modeling approaches can alsobetter track trip chaining behavior and the selection of non-motorized modes resulting in bettermodel calibration than current practices (US DOT, 1995).

9 The Transportation Research Board (TRB) recently documented the evidence for inducedtravel and concluded that there is some e ect but was inconclusive on whether this implied anyimpact on the environment, especially air quality (Transportation Research Board, 1995). Thisstudy concluded that restraining growth in highway capacity would result in minor, if any, im-provements in air quality. The conclusions of the TRB committee suggest that pricing and landuse policies are more e ective at achieving long run impacts at improving air quality compared toa policy of restraining growth in road capacity .

10 However, land use strategies can be underminedby the construction of new highway capacity and pricing policies are to some extent an ac-knowledgement that induced demand is a major problem (and is one of the responses beingconsidered in the UK).The debate over induced travel has largely centered around the potential increased social costsfrom generated tra c. Another perspective is to consider the social bene ts derived from short-ening trip times and allowing more people to travel when and where they want.