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Lorries cause more damage to roads than cars

Philippa Edmunds's picture
Sign: no lorries

Local roads bear the brunt of potholes, yet it is heavy goods vehicles, not cars, that are most to blame, argues Philippa Edmunds.

Most of our local roads were never designed to take either the weight or the volume of heavy goods traffic for which they now have to cater and that is a key reason why many show signs of structural failure. These points were highlighted recently by the Local Government Association in a press release with the headline 'Pothole levels likely to surge warn councils because of spike in number of heavy lorries'.

The Freight Transport Association rushed to respond, claiming that 'Lack of investment, not freight transport, [is] to blame for potholes'. According to the Freight Transport Association, "Larger lorries do not cause increased damage to the road surface – in fact, they have more axles which spread payloads more evenly."

But contrary to their statement, lorries do cause far more damage to foundations and structures of roads than cars because the damaging power rises exponentially as weight increases. This is called the Generalized Fourth Power Law

The Generalized Fourth Power Law is the most commonly agreed method to approximate the relative impact of vehicles on roads: the damage caused to the structure or foundations of a road is related the axle weight of the vehicle by a power of four. 

This means that a six-axle, 44-tonne truck is over 138,000 times more damaging than a typical, small, 1 tonne car (such as a Ford Fiesta) with two axles. 

In actual fact, even before the Fourth Power Law is applied, the comparison in weight per axle of 0.5 tonnes for a Fiesta and 7.33 tonnes for a large truck is stark. So, more axles do spread the weight – but nowhere near enough to support the Freight Transport Association's claims.

(Interestingly, Transport Research Laboratory suggests that the Sixth Power Law could be applied to weak pavements.)

The structural damage caused to road foundations by heavy goods vehicles is expensive to fix. Motorways are constructed to a higher specification than local-authority-run roads to cater for heavy goods vehicles but it is the latter which make up almost 98% of our network.

Furthermore, lorries are not paying for most of the road maintenance costs with the bill for these repairs largely falling to taxpayers. Research carried out for the Campaign for Better Transport using DfT criteria found that HGVs pay less than a third of the costs associated with their activities, in terms of road congestion, road collisions, road damage and pollution which equates to an annual subsidy of around £6.5 billion. These conclusions are in line with a MDS Transmodal study in 2007 which found a very similar amount of underpayment: £6 billion.

So for socio-economic and environmental reasons, Campaign for Better Transport hosts the Freight on Rail campaign, promoting rail freight as a safer, less polluting mode which reduces road congestion and road damage.