Anticipating vehicle traffic increase on improved inter-urban roads: evidence from three decades of transport projects in developing regions

ABSTRACT

Consideration of the traffic generated by inter-urban road investments is important for assessing their economic feasibility and external costs and for designing sustainable road maintenance strategies. While the literature and evidence on generated traffic is growing, it has almost exclusively focused on advanced economies. In contrast, readily available methodologies for predicting the generated traffic impacts are lacking in low and middle income countries (LMICs), where most of the future road investments will be made and where detailed travel demand models and data that would be needed to feed them are often not available. This study attempts to fill this gap by specifically focusing on observed traffic growth and its drivers in developing countries. After reviewing the literature on generated traffic modelling, it presents empirical evidence on the characteristics and outcomes of inter-urban road projects implemented over the last three decades across 68 LMICs. The study quantifies the statistical association between, on the one hand, the travel time or vehicle operating cost (VOC) savings resulting from the road improvement and, on the other hand, the observed short-term growth in traffic on the project roads. Controlling for a range of macro-economic and project-level attributes, both travel time savings and VOC reductions are found to have a statistically significant, positive association with observed short-term traffic growth of a magnitude that much exceeds the elasticities reported in developed country literature. Population growth in the project country/State/province during the project implementation period is also found to have a large, positive association with traffic growth, while per capita income growth is found to have a marginal effect. Toll roads (albeit representing a small share of the sample) are found to have statistically significantly lower observed traffic growth compared to non-toll roads even after controlling for the other project characteristics.

KEYWORDS:

• Generated traffic
• induced traffic
• elasticity
• inter-urban roads
• highways
• multilateral Development banks

Acknowledgements

The authors would like to thank Rodrigo Archondo-Callao, Shomik Mehndiratta, and Martin Humphreys at the World Bank for valuable comments and suggestions on earlier drafts.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Notes

1 The Notes define “induced traffic” as the additional traffic (in person or vehicle kilometres) that has been induced by the project through mode changes, destination changes, trip re-timing, trip frequency changes or new trips associated with different land uses.

2 The definition of ‘inter-urban’ vs. ‘rural’ roads is not consistent across all countries; many of the ‘national-importance’ roads that were included in the analysis carry relatively low traffic volumes (<300 vehicles per day).

3 Among other sources, the Department of Transport hosts an online database on Annual Average Daily Flow and traffic data for every junction-to-junction link on the “A” road and motorway network in Great Britain (Department of Transport, 2014b).

4 In individual cases, the data in the project documents is presented for an aggregate of several roads. These are considered as a single observation in the analysis dataset.

5 Correlation coefficient of 0.58 for the 122 roads for which both variables are reported.

6 As a comparison, a comprehensive, UK-focused review by Hanly, Dargay, and Goodwin (2002) found that a 10% increase in income is associated with a short-run growth in traffic volume of about 2%.

7 In a comprehensive review covering 151 road schemes in the UK, SACTRA (1994) and Goodwin (1996) found that, on average, the observed traffic on the improved roads was about 10% higher than had been forecast assuming no induced traffic.