Carbon Policies Targeting Road Transport: Is There a Safety Consequence?

Carbon Policies Targeting Road Transport: Is There a Safety Consequence?


Reza Tolouei, AECOM


This study investigate whether there is any interactions between environmental policies and safet outcomes within the vehicle fleet, imposed by vehicle design.


Environmental and safety goals in transport can interact in several different ways. One of these potential interactions is found within the vehicle fleet where fuel economy and safety impose conflicting requirements on vehicle design. Larger and heavier vehicles have a better safety performance during a crash. On the other hand, they are associated with higher levels of CO2 emissions. This issue has generated debate amongst researchers and policy makers when formulating policies to improve the environmental performance of the road transport system. An extensive review of literature reveals that arguments have often been based on either little research evidence, or research that has inadequacies in the applied methodologies (Tolouei, 2011).

For an individual vehicle in fleet, vehicle mass is a significant contributor to both carbon emissions and safety performance of the vehicle with a potentially interacting effect. Although it is generally agreed that a reduction in vehicle mass in a vehicle fleet results in a reduction in overall fuel consumption and carbon emissions, all other factors remaining constant, there are inconsistencies in the estimates as a result of the shortcomings in the methodologies used. This is addressed in an earlier study (Tolouei and Titheridge, 2009) where the partial effects of vehicle design features on fuel consumption rate were estimated. On the other hand, there are disagreements concerning the effects of such changes within a fleet on safety because of the complicated effect of vehicle mass in two-car crashes. Similarly, a separate study investigates the effects of vehicle mass on injury outcome in two-car crashes (Tolouei et al., 2013).

The findings on the effects of vehicle design, particularly mass, on vehicle safety performance and vehicle fuel consumption and carbon emissions, as referred above, could be used to investigate whether there is any conflict in safety and environmental goals as a whole in a vehicle fleet with given characteristics. This is addressed in this study by investigating likely safety and environmental consequences of changes in mass distribution within the vehicle fleet. Passenger cars in Great Britain are chosen as the vehicle fleet to study. The effects of a number of hypothetical mass distribution scenarios on overall fleet fuel economy and safety were estimated. Three hypothetical alternative scenarios (i.e. fleet downsizing, symmetric reduction in fleet diversity, asymmetric reduction in fleet diversity) were defined according to their mass distribution. For each scenario, two cases were examined: a first case where vehicle mass is changed while vehicle size is maintained, and a second case where vehicle size is also changed accordingly with vehicle mass. To investigate the safety and environmental consequences of these different mass distributions, a new methodology based on disaggregate cross-sectional analysis of vehicle mass within the fleet was used.

The results generally show that an informed change in the mass distribution of vehicles not only imposes no trade-off between the fuel economy and safety goals, but also could lead to a desirable outcome in both aspects. According to the results, the most favourable scenario for fleet fuel economy is shown to be the uniform downsizing scenario. Depending on the scale of reduction in mass, considerable reduction in the overall fuel consumption and carbon emissions could be gained. This will be accompanied by no increase in the number of casualties as a result of the change in vehicles’ masses if the size of vehicles is maintained (i.e. a change in vehicle design towards using lighter materials).

Such scenarios are unlikely to be repeated exactly in reality as a number of other contributing factors including vehicle ownership and vehicle usage pattern are also likely to change over time; however, they provide the necessary basis to formulate policies related to the vehicle fleet that aim at reducing overall fuel consumption or number of crash injuries and fatalities where no adverse impact on either side would be expected. The study includes a discussion on various policy options which could influence the mass distribution of vehicle fleet, and their likely consequences (e.g. CO2 emission regulations, vehicle taxing policies, fleet downsizing policies, and policies that target behavioural changes). Some recommendations are made, and direction of further work in this area is suggested.


Tolouei, R., 2011. Interactions between environmental and safety performance in vehicle design. PhD Thesis. University College London.

Tolouei, R., Titheridge, H., 2009. Vehicle mass as a determinant of fuel consumption and secondary safety performance, Transportation Research Part D, 14(6), 385-399.

Tolouei, R., Maher, M., Titheridge, H., 2013. Vehicle mass and injury risk in two-car crashes: a novel methodology”, Accident Analysis and Prevention, 50, 155-166.


Association for European Transport