Can we build our way out of congestion?

I gave a talk to the Transport Economists Group in London on 28 October 2015. Much of the material covered can be found in recent articles on this website. One new theme is set out here.

The Government has announced its Road Investment Strategy that commits £15 billion expenditure over the next five years. One stated aim is a ‘free-flow core network, with mile a minute speeds increasingly typical’. How realistic is this?

Let’s consider the past pattern of travel behaviour that has been tracked over the past forty years by the National Travel Survey. Average travel time has stayed steady at about 370 hours a year, or an hour a day, a finding that holds true for all settled human populations. What has changed over the period is the average distance travelled, which increased from 4500 miles a year in the early 1970s to 7000 miles in the mid 1990s, since when this has ceased to grow. Increased distance in unchanged travel time is the result of investment in the transport system that has permitted faster travel – private investment in cars, public investment in roads and railways.

Not time savings

People have taken the benefit of investment by travelling further to more distant destinations, not by saving time in reaching unchanged destinations. This is contrary to what transport economists suppose when they estimate the main benefit of investment as time savings, valued for the extra work or leisure supposedly made possible. In reality, people travel further to have more opportunities and choices. For instance, by travelling faster on the journey to work, you have more choice of jobs accessible from where you live in the time you allow yourself for travel, more choice of homes accessible from your workplace, and similarly more choice of shops, schools and so forth.

So people take advantage of road improvements that permit faster travel to make longer trips as part of their daily routine. This is particularly the case in areas where demand for housing exceeds supply, creating an incentive to travel further in search of affordable properties.


Daily travel is an important component of traffic on parts of the Strategic Road Network (SRN). Congestion on this network arises near to populated areas, where local users interfere with long distance users. Half the traffic on the M25 is local. Remote from populated areas, the traffic generally flows freely. The conventional response to congestion on the SRN is to add capacity – an extra lane, conversion of the hard shoulder, or an improved junction. Conventional economic appraisal involves multiplying small time savings from such capacity increases by a large number of vehicles and by standard values of time to generate monetary benefits that can be compared with the costs of the extra capacity, to assess value for money.

Small savings

The time savings per vehicle are quite small. The Highways Agency (now Highways England) carried out evaluations of around 120 completed major improvements and found the average time saving to be three minutes at times of peak congestion. There has been debate about the value of such small time savings. One view is to disregard these as too small to change behaviour. Against that, it is argued that small time savings can accumulate as more improvements are implemented, so in logic all need to be counted.

While three minutes is too small to matter for a long distance trip, it is not insignificant for a local journey. So if we add carriageway to a congested section of the SRN, it is the local users who take advantage of the faster travel to make rather longer trips, particularly for greater choice when they change jobs or move house. These lengthier trips generate extra traffic,  which restores congestion to what it was previously. Long distance users are no better off.

Induced traffic

This extra traffic is what is known as ‘induced traffic’, about which there used to be debate – did it arise and if so why? We can now see that induced traffic is the extra traffic that arises because people take the benefit of road improvements that allow faster travel as more opportunities and choices at greater distances, consistent with the evidence of the National Travel Survey, rather than as time saved.

Occurrence of induced traffic is the basis for the maxim: ‘You can’t build your way out of congestion’, which from experience we know generally to be true. This is what transport ministers at one time used to say, when they did not have a big budget for road construction. Current ministers tend to speak rather vaguely about new road schemes ‘creating opportunities for hardworking people across the nation and driving economic growth’, but no doubt hoping that congestion would be lessened, as is seemingly implied by the time saving rationale.

What road construction can achieve is to make land accessible for development. But this needs to be led by planners and developers identifying sites suited for development that are commercially attractive. If such sites require improved road access, then this should be a candidate for funding, whether from a local transport investment budget or a national funding programme, subject to a value for money test. Such local initiatives fit with devolved funding, not as part of a national Road Investment Strategy for which local development is inadvertent or incidental.


If we can’t build our way out, what do we do about congestion? Surveys of road users find that the main perceived problem arising from congestion is unreliability, rather than increased time taken. We can tackle the unreliability problem by providing road users with good predictive travel time information before they set out, so reducing uncertainty in arrival time. This is becoming increasingly possible through digital technologies, which are far more cost effective than traditional civil engineering technologies in meeting the needs of road users. In Britain, we are familiar with roadside variable message signs predicting the time to the next junction – although this tends to be too late to be of much use.

One example of useful predictive travel time information is found at Seattle, where you can input the postcodes of your home and workplace, and the time you want to arrive at work, to be advised of the time to leave home to arrive on time nineteen times out of twenty. A more ambitious example has been in operation in Nordrhein-Westfalen in Germany, where a simulation model of the autobahn network predicted journey times for any kind of trip (although this website seems no longer to be active). Such simulation models may be expected to improve their predictive accuracy as computers become more powerful, faster and cheaper and can process increasing amounts of input data.

Two kinds of driver

Predictive journey time information can be used by the two kinds of driver on the roads. Those who need to be at their destination at a particular time will know when they need to set out – whether to get to work or a meeting, or deliver time-critical goods. Those who are more flexible may be able to use such information to avoid peak traffic – for instance when on shopping or leisure trips or visiting friends. The more the flexible drivers can avoid peak traffic, the less congestion for those who have to be on the roads at that time – which is win-win.

Anecdotal evidence of the usefulness of digital technologies is to be found in Just-in-Time delivery, offered by efficient road freight haulage businesses who understand the road network well and can manage their vehicle fleets to perform rather precisely. For instance, a haulier working for a supermarket business to deliver from the central warehouse to the stores may be contracted to deliver within 30 minutes time slots, and can do so.

Digital futures

These digital technologies need to be made generally available. Highways England has an important role. The investment case for digital technologies requires monetary values for journey time reliability, which might be available from the latest Department for Transport research on the value of time. The investment case also requires information about drivers’ response to predictive information about journey times, which is researchable, albeit not a static situation since positive responses are likely to increase with familiarity.

A further consideration is the information currently made available by specialist providers such as TomTom or general providers such as Google. It is not clear how reliable is this information or what impact it is having on the functioning of the road network. Nevertheless, there is scope for collaboration between Highways England, which has an interest in the overall efficiency of the SRN, and the private sector businesses that provide information to individuals for reward, directly or indirectly.


It is a step forward that Highways England’s recent Concept of Operations recognises the importance of maximising the throughput of people and goods through initiatives such as smart motorways and Intelligent Transport Systems ‘to squeeze every drop of capacity out of what we have’.

So while we can’t build our way out of congestion,  we can manage the problems arising from congestion far more effectively. But to do that, we need a substantial reallocation of planned expenditure within the Roads Investment Strategy. The £15 billion spend over the next five years includes some ring-fenced funds for Innovation (£150m) and Growth & Housing (£100m). However, these earmarks are a tiny proportion of the total, with the bulk of spend devoted to traditional civil engineering work aimed at increasing capacity – a very twentieth-century approach that is not appropriate for the digital twenty-first century.





  • […] Prof Metz points out that a true transport choice (and change) is badly represented in current models. In his book Peak Car he argues that there is one true constant in transport behaviour and that is travel time. It is the ease of driving that expanded our travel distance, enabled through scattered land use planning and motor-centric highway engineering. The good news is, we planned ourselves into car dependency and we can plan ourselves out of it. But to do this we need supporting appraisal and evaluation processes. You can read more in Prof Metz’s book Peak Car and on the Peak Car website here. […]

  • […] reduction and the benefits of improving connectivity between cities. We know from experience that we can’t build our way out of congestion – any increase in road capacity in populated parts of the country attracts additional commuting […]

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