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Environmental effects
Health effects
Guidelines for policy makers
Psychological restoration and car use
Common assumption: increases in urban population density---as promoted by the compact cities and mixed-use development---automatically reduce the amount of travel. Studies in urban Sweden cast doubt on this assumption.
Amount of travel is better predicted by looking at car availability, which increased most dramatically in the smaller towns and rural areas. Medium-sized Swedish cities (50000--200000 inhabitants) actually saw a decrease in car use as more people took up bicycling for their daily commutes.
Economic factors strongle affect people's access to cars. Policy makers should therefore be relieved to know that a greater effect on overall mobility can be achieved using economic incentives (e.g., environmental surcharges) than by promoting high-density development.
``Total volume of mobility, in terms of distance traveled, is mainly influenced by the available speed.'' cf. Marchetti constant---1 hour on average per person per day for all trip purposes.
Factors affecting the individual's choice of travel mode and daily itinerary:
Prisoner's dilemma/Tragedy of the Commons: from the individual's perspective, the best payoff is obtained by ``defecting'' even though society as a whole would be better off if everyone cooperated.
Positive consequences of car use are felt directly and immediately by the individual car user, while negative consequences affect future generations and can't be traced to the travel decisions of any individual car driver.
Lack of viable alternatives to the car, combined with distance to work and other destinations, explains why many car users do not change their behavior even when they express willingness to do so.
Affective and symbolic aspects of care use and ownership are difficult to give up after an individual has tied up her identity to her car and the lifestyle it enables.
It might be better to never have had control than to have lost it.
-- Murphy, L.R. (1988), Workplace Interventions for Stress Reduction and Prevention, In Causes, Coping and Consequences of Stress at Work (C.L. Cooper and R. Payne, eds.), Wiley, Chichester, 301--342.
Car use develops into a habit, as the situational cues which trigger the choice of a car over other transport modes become more general.
Experiment by G�rling et al. (2001):
1. Subject acquires car-use habit for trips longer than walking distance.
2. Distance to the destination is decreased.
3. Car-use habit persists.
G�rling et al. infer that subjects are distorting the evaluation of outcomes, either by not attending to feedback or by misinterpreting it.
Among the environmentally-relevant behaviors studied in conjunction with personal beliefs and values, travel mode choice was least affected by personal ecological norms (Harland et al. 1999). Easier behavioral changes (use of unbleached paper, use of energy-saving light bulbs, and turning off the faucet when brushing teeth) were better explained by introducing personal ecological orientation as a possible predictor.
Personal ecological norms might end up being suppressed in order to minimize dissonance (e.g., after listening to the perspectives of other car users in group discussions, as in the 1997 study by Steg and Vlek).
Policies aimed at changing people's travel behavior are called travel demand management (TDM) measures, and they admit various possible subclassifications.
Litman's breakdown (2003):
breakdown by May et al. (2003):
More broadly we can distinguish between:
Attributes of TDM Meaure:
Outcome Variables:
political feasibility, effectiveness, public attitudes
Market-Based mechanisms
Idea: raise costs so that the congestion externality is internalized
Popular among politicians recently, esp. for generating revenue
Questioned among researchers who doubt its claimed first-best nature as a congestion relief measure
Price elasticities of demand have to be taken into account.
Regulatory mechanisms
Idea: restrict the available travel behaviors so as to reduce the negative consequences of transport mode choices.
Decisions on road construction historically follow a ``predict-and-provide'' approach.
Hansen and Huang (1997) conclude, from five-year elasticity of vehicle travel with respect to highway lane miles, that most of the trips using a new road in California are trips that would not have occurred had the road not been built.