car driving through forest

Counting the full costs of transport choices

6 February 2023
From our ‘Thinking outside the box’ series
John Rose is the Neil Smith Chair in Sustainable Transport Futures. Here, he discusses how calculating the environmental impact of transport involves more than simply calculating CO2 emissions.

Consumers in general are often very poor at understanding the full implications of their choices, particularly with respect to things they cannot directly observe themselves. When purchasing a cup of coffee for example, most consumers will be aware of the potential impact single use plastic lids have on the environment. Very few however will take the time to understand the origin of the coffee beans used to make their coffee, determine the potential environmental effects the production process may have at these locations (e.g., issues with possible deforestation, soil erosion and water contamination), or seek information as to whether or not modern-day slavery was employed anywhere along the supply chain. Unfortunately, the consequences affecting other parties arising from the industrial and commercial activities involved in delivering a coffee to the end consumer, are not likely to be reflected in market price of the coffee paid by the consumer. Economists have a word for such impacts:  externalities.

When it comes to transportation choices, the general public are far more likely to believe that they have a better understanding of the consequences their decisions have on others, particularly with respect to the impact their choices have on the environment. In part, this is due to the fact that transportation as a sector tends to attract a disproportionately high level of media attention, as well as government intervention. For example, news coverage concerning electric vehicles has increased substantially over the past four to five years, whilst the government mandated that all new petrol vehicles sold since 1986 use unleaded petrol before implementing a nationwide ban on the sale of leaded petrol in 2002. Independent of one’s belief as to whether or not climate change is real, or caused by human activities, it is highly unlikely that any given traveller would fail to understand that vehicles relying on internal combustion engine technology as the primary power source generate emissions, particularly in the form of CO2 gases.

However, the ecological advantages of so called environmentally friendly vehicles, particularly electric vehicles, over internal combustion engine vehicles may not be as clear cut as many travellers believe. Considerable debate exists as to what, if any externalities are associated with electric vehicle manufacturing and use. Whilst consumers may be aware that the electricity used to charge batteries may come from unsustainable energy generation sources such as coal, there is growing consensus that other lesser-known externalities arise from simple use of vehicles on the road. One study conducted in London for example, monitored air pollution levels along 65 roads over a ten-year period and found that across many routes, air quality improvements resulting from the use of lower emission vehicles were outweighed by increases in particulate matter arising from wear on tires vehicle brakes, and roads in general (Font and Fuller 2016).

Consider the very act of braking. Most braking mechanisms involve some form of friction system that converts the kinetic energy of the vehicle into friction heat. Wear and tear of brake pads and brake rotors in such systems is unavoidable, resulting in material deposits along the roadway as well as airborne particulate matter (Lyu et al. 2020). Such particles largely consist of transition metals which have been shown to be highly toxic. Aside from environmental impacts, air pollutants from vehicle braking and road and tyre wear have been shown to increase cardiovascular diseases, and may also contribute to increases in Alzheimer’s disease, asthma, acute respiratory infections, cancer, as well as a whole raft of other medical conditions.

Unfortunately, some estimates suggest that particles derived from braking and tyre wear may be up to 1,000 times the acceptable pollution limits set by the EU and contribute up to 20 percent of all traffic emissions (Grigoratos and Martini 2015). Worse yet, moves towards greater electric vehicle uptake may exacerbate the issue. This is because one of the major causes of brake and tyre wear is vehicle weight, and unfortunately, electric vehicles tend to be far heavier than other types of vehicles on the road. As such, whilst adopting policies to promote electric vehicle uptake and use may on the surface appear to represent an environmentally responsible action, without understanding the full set of externalities of such technology adoption, it is possible we may be making things worse.

There currently are no policies related to controlling these types of emissions. What is known is that braking at speed from 50 km/hr results in almost twice as much particle emissions than braking from 30 km/hr, hence changing speed limits in urban settings may be one possibility. Developing new braking systems and adopting new tire technology may also provide some benefits. Of course, the most realistic possible solution is to reduce traffic volumes outright.


Font, A. and Fuller G.W. (2016) Did policies to abate atmospheric emissions from traffic have a positive effect in London? Environmental Pollution, 218, 463-474.

Lyu, Y., Ma, J. Ma. J., Hedlund Astrom, A. Wahlstroom, J. and Olofsson, U. (2020) Recycling of worn out brake pads ‒ impact on tribology and environment, Scientific Reports, 10, 8369,

Grigoratos, T. and Martini, G. (2015) Brake wear particle emissions: a review, Environmental Science Pollution Research International, 22(4), 2491-2504.