Analysing the role of traffic volume as mediator in transport policy evaluation with causal mediation analysis and targeted learning

Yingheng Zhang, Haojie Li and Gang Ren

Transportation Research Part A: Policy and Practice, 2025, vol. 192, issue C

Abstract: Traffic volume has often been analysed as the outcome of interest in transport policy evaluation. However, its role as mediator lying in the causal pathways between policies and other outcomes has rarely been explored. We investigate this issue by using a targeted learning-based causal mediation analysis approach. Compared to the traditional approach that has been used in transport research, namely the Baron-Kenny multiple-regression approach, the causal one incorporating potential outcomes has clearer causal definitions and interpretations. Also, targeted learning has higher functional flexibility by enabling the use of supervised learning algorithms. Simulations indicate that targeted learning outperforms the traditional approach in complex settings with nonlinearities and interactions. We present an empirical example, quantifying the direct effect of the London Cycle Superhighways (LCS) on traffic speed, and the indirect effect via traffic volume as mediator. Our results indicate that the installation of LCS has reduced motor traffic along the routes. The average causal effect on annual average daily traffic (AADT) relative to the AADT in the pre-intervention period is − 9.2 %. Regarding the direct and indirect effects, we find that LCS has a negative direct effect on traffic speed, which might be due to less space available for motor vehicles, while LCS can increase traffic speed via reducing the amount of motor traffic. The direct effect on traffic speed relative to the speed in the pre-intervention period is − 2.0 %, whereas the indirect effect is + 1.3 %. As a result, the total causal effect on speed is small.

Keywords: Policy evaluation; Mediation; Potential outcomes; Causal machine learning; Cycling (search for similar items in EconPapers)
Date: 2025
References: Add references at CitEc
Citations:

Downloads: (external link)
http://www.sciencedirect.com/science/article/pii/S0965856424004178
Full text for ScienceDirect subscribers only

Related works:
This item may be available elsewhere in EconPapers: Search for items with the same title.

Export reference: BibTeX RIS (EndNote, ProCite, RefMan) HTML/Text

Persistent link: https://EconPapers.repec.org/RePEc:eee:transa:v:192:y:2025:i:c:s0965856424004178

Ordering information: This journal article can be ordered from
http://www.elsevier.com/wps/find/supportfaq.cws_home/regional
https://shop.elsevie ... _01_ooc_1&version=01

DOI: 10.1016/j.tra.2024.104369

Access Statistics for this article

Transportation Research Part A: Policy and Practice is currently edited by John (J.M.) Rose

More articles in Transportation Research Part A: Policy and Practice from Elsevier
Bibliographic data for series maintained by Catherine Liu ().