Smarter Cambridge Transport

Does building more roads reduce congestion? No, and here’s why.

Does building more roads reduce congestion? The answer seems obvious. More road space means less congestion. But experience and behavioural psychology tell a different story. More road space just increases the distances people travel.

Here’s why. People have a relatively fixed budget of travel time: on average 377 hours/year (within the UK), up slightly from 353 hours in 1973. Motorways and lower motoring costs (relative to earnings) have led us to ‘spend’ this travel time budget on travelling nearly 50% further than in 1973: longer commutes, more dispersed networks of family and friends, and venturing further afield for shopping, leisure pursuits and holidays. (Data from the National Travel Survey 2018.)

Another constant is people’s tolerance to congestion. Pick any decade since the 50s and you’ll find news articles moaning about the “intolerable” congestion. The thing is, enough people do tolerate congestion that they make it inevitable – what mathematicians call an ‘equilibrium state’. Typically, within five years after a road is upgraded, the traffic volume increases to a level where congestion is similar to before the upgrade.

In the case of piecemeal road upgrades, congestion is more often displaced than (temporarily) reduced. For instance, dualling the A10 from Ely will increase congestion on Milton Rd, the A14, A428 and M11.

This is why widening roads does not reduce congestion: it just increases the distances people travel. That’s not the outcome people stuck today in congestion actually want.

Is more travel good for the economy? In most respects, no: someone who commutes 50 miles to work is no more economically active than someone who commutes 5 miles. Do you spend more on food or clothes simply because you’ve driven further to buy them? However, there is one respect in which travelling further has an economic benefit: it gives employers a larger catchment pool to recruit from, increasing productivity and facilitating business expansion. This is the main economic justification for investment in roads and rail.

Politicians hear their constituents complaining about congestion; they hear businesses complaining about recruitment. They put two and two together and conclude the answer is to increase road capacity. Economists dutifully provide a cost-benefit analysis to support their decision.

So, widening roads does not reduce congestion in the long run. Any reduction is temporary, negated by increased congestion elsewhere, or both. There is an economic benefit of allowing some people to travel further to work, but that benefit is unjustifiably small. It’s much better to use existing road space more efficiently.

We’ve been slow to recognise it but, alongside our time and money budgets, we also have a carbon budget to manage. Most carbon emissions derive from converting fossil fuels to energy in vehicle engines, boilers, power stations, blast furnaces and so on. That accounts for 80% of our total energy consumption. Replacing these with carbon-free alternatives will take decades. So, for now, we have no choice but to reduce our energy consumption.

That means travelling less and, instead of driving, using more energy-efficient transport modes: trains, buses, cycles and walking.

More people taking buses gives us the twin benefits of reducing carbon footprints and reducing congestion. Here’s how.

A bus or coach carrying 48 passengers at 30mph requires about 35m of road to maintain a safe distance from the vehicle in front. Put those passengers into single-occupancy cars, and they need about 1.4km of road (nearly a mile). The same number in a bus or coach travelling at 60mph requires 70m of road; in cars, 2.6km. So, getting more people onto buses increases road capacity enormously – in terms of the number of people moved per hour, which is what actually matters. Those passengers also consume about a tenth of the energy they would if they were driving.

The benefits from walking and cycling more are even greater: negligible additional energy consumption, and high space-efficiency, as well as the health improvements.

Some travel choices may be discretionary (e.g. where you holiday), but many are determined by government policies on development and transport. Widening roads can only increase people’s carbon footprints. So, the only sane and responsible strategies for politicians to pursue are to provide more public transport – lots more; to enable more walking and cycling; and to promote higher density, mixed-used development that reduces the distances people have to travel to work, schools, shops and leisure pursuits.

This article was first published in the Cambridge Independent on 2 and 9 October 2019.

Edward Leigh

Edward Leigh is the leader of Smarter Cambridge Transport, chair and independent co-opted member of the Cambridgeshire Police and Crime Panel, chair of the South Petersfield Residents Association, business owner, consultant, and occasional blogger about making the world and Cambridge a better place to live.


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  • I think a key problem is that buses are seen as low quality transport, as well as offering suboptimal routes and timings. Rather than have park and ride, which only solves congestion in urban centers, we need people to catch the bus, or coach, for the full journey from near home.
    Can we not have little pods that carry one person, with single-pod shuttles for short journeys, and then load and unload pods onto larger carriers for longer journeys?