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Do New Roads Generate Traffic?

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Until relatively recent years the traffic and economic appraisal methods used for trunk road schemes have assumed that reassignment (i.e. making the same journey by a different route) was the only significant impact on trip making behaviour resulting from trunk road investment. This assumption is no longer generally accepted, as empirical and theoretical research suggests that, both in the short and longer term, a wider range of responses takes place in addition to reassignment. While the extent of these responses has been unclear, theory suggests that in some circumstances they could have a significant impact on the economic benefits of schemes. These responses (collectively referred to as induced traffic effects) include: rescheduling of trips to take advantage of improved conditions at peak periods; increasing frequency of trips; decreasing vehicle occupancy; switching between public transport and private vehicles (mode shift); travelling to new destinations; making entirely new vehicle trips and changes in the patterns of land use or car ownership. The opening of the M60 Manchester Motorway Box (MMB) completed one of the last major links in the UK motorway network and the scale of the new road scheme meant that its completion in October 2000 gave an opportunity to collect data concerning the extent of induced traffic. Preliminary studies undertaken in 1996 recommended and designed a programme of Before (1999) and After (2003) surveys comprising large-scale Roadside Interviews and public transport surveys. Additionally, a large-scale home interview survey was conducted in 2002. Together, these surveys provide a large and comprehensive data base for analysis and modelling.

In 2005, RAND Europe was commissioned to develop models to quantify the magnitudes of the different induced traffic effects. Because the size of some of these induced effects was not known beforehand, we adopted an approach which made use of the full variance present in the data by preserving the individual observations from the intercept surveys and home interviews. The pooling of these two very different types of data added to the complexity of the modelling, and complex weighting procedures were required to take into account that the intercept surveys are biased towards long trips. The final models were of high quality, in terms of the statistical reliability of the parameters, the expectation regarding the relative sizes of parameters, the ability of the model to predict observed trip lengths and the resulting model elasticities.

Model runs were undertaken for two travel purposes: commuting and other travel, using the travel time changes observed from the Manchester Motorway Box to examine the size of the induced traffic, and the relative impacts of different behavioural responses.

This presentation describes the study background, the available data, and, briefly, the modelling estimation framework, before going on to present the results from applying the model to give an indication of the relative size of the induced traffic effects as a result of the completion of the Manchester Motorway Box. The results show a significant increase in traffic between areas served by the new road and reinforce the need for transport planners to take account of induced traffic effects when considering the benefits of new transport infrastructure.

This talk is part of the Statistics series.

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