Recent revisions to the road traffic statistics appear to show that there has been a substantial growth of motor vehicle traffic on GB minor roads in recent years, from 108 to 136 billion vehicle miles between 2010 and 2019, an increase of 26%. Traffic on major roads rose from 197 to 221 bvm over the same period, an increase of 12%.  (DfT Road Traffic Statistics TRA0102).

Road traffic statistics are based on a combination of automatic and manual traffic counts. Major roads are well covered in that traffic in all links is counted on typical days, although not every link in every year. Given the vast number of minor roads, however, it is only possible to count traffic at a representative sample of locations every year, and the observed growth is applied to minor road traffic overall. Estimates from a fixed sample may drift over time such that the sample becomes less representative of the changing minor road network. To account for any errors incurred in the fixed sample, the sample is revised through a benchmarking exercise every decade, involving a much larger sample of locations.

The most recent minor roads benchmarking exercise was published in 2020, based on 10,000 representative locations. Overall, the benchmark adjustment for 2010-2019 was 1.19, which is the factor to be applied to 2019 data from the original sample to bring this to the observed traffic level. Data for minor roads traffic for intermediate years are adjusted pro rata, to avoid a step change in the reported traffic data. There is significant regional variation in the adjustment factor, from 1.35 for Yorkshire to 1.09 for East of England, with London at 1.32. For B roads the factor is 1.25, for C roads 1.17; while for urban roads, 1.22, and for rural roads, 1.15. Applying the regional weightings yields an increase in traffic on minor roads of 26%, as noted above, whereas the increase based on the original sample would have been 6%.

The previous benchmarking exercise published in 2009 found a smaller overall adjustment factor of 0.95, with a regional range of 0.81 to 1.08.

The substantially greater adjustment required following the recent benchmarking, compared with the earlier exercise, suggests that there has been a real change in use of minor roads, beyond errors arising from drift in the sample. Importantly, had the increase in minor road use been spread evenly across the national road network, the traffic estimation based on the sample would have been close to that from the benchmark exercise. Hence the major difference between sample and benchmark indicates considerable heterogeneity of minor road traffic growth. Moreover, the fact that the sample failed to detect the traffic growth suggests either that the process for establishing the sample was deficient in some way, or that significant changes occurred in use of minor roads over a decade.

DfT statisticians have created a revised minor roads representative sample (4,400 locations) from the latest benchmark data, which will be used for the coming decade. It would be desirable to have comparative analysis of the previous and the new samples, to gain insight into what has been happening on the minor road network. Regrettably, the statisticians only report findings, and do not attempt to explain them, which leaves uncertainty as to the nature and cause of the reported changes to traffic volumes. The representative nature of the new sample must be questionable if the reasons for the failure of the previous sample to reflect reality are not understood and addressed.

Transport for London has recognised this uncertainty. The recent Travel in London Report 13 discusses the implications of the minor roads traffic correction (p92). The revisions mean that, for 2018, the DfT estimate of vehicle kilometres was 20% higher than previously reported last year (and included in Travel in London Report 12). The previous estimate suggested a fall of 1.8% in vehicle kilometres in London between 2009 and 2018, whereas the revised series now suggests an increase of 17.9% over the same time period, this change wholly arising from revisions to the minor road estimates. TfL notes that it is currently working through how the DfT have made this assessment, and also what this could mean for London data sets. For the moment, TfL is relying on its own traffic monitoring data, although it does not report traffic on minor roads separately.

The National Travel Survey could provide a cross-check on the traffic data. Average distance travelled by car/van driver decreased from 3388 miles per year in 2010 to 3198 miles in 2019, a decline of 5.6% (NTS0303). The GB population grew from 60.95m in 2010 to 64.90m in 2019, an increase of 6.5%. The net increase in car use of about one percent is inconsistent with the new road traffic statistics which show an increase in traffic for all roads of 17% over the same period. The NTS employs a fresh sample of respondents each year, so sample drift should not be a concern. However, it is not clear that the travel diary technique would pick up rerouting to minor roads, given that respondents are asked to provide their own estimates of distance travelled for each trip.

Possible causes of increase in traffic on minor roads

One factor contributing to the growth of traffic on minor roads is the increase in van traffic, including that arising from the growth of online shopping with home deliveries. The number of vans (light commercial vehicles) registered in Britain increased by 28% between 2010 and 2019. Total van traffic increased by 34% over this period, with an increase of 49% on urban minor roads compared with 10% on urban ‘A’ roads, although ‘delivery/collection of goods’ was less important in respect of journey purpose than ‘carrying equipment, tools or materials’. However, in 2019 van traffic amounted to 15% of traffic on urban minor roads, and 19% on rural minor roads, cars being responsible for 82% and 78% of traffic respectively. So, the growth of van traffic on minor roads is responsible for only part of the overall traffic growth on these roads.

Another possible explanation of the apparent large growth of traffic on minor roads is the widespread use of digital navigation (satnav) that offers routes that take account of traffic conditions and estimated journey times. Such devices make possible the general use of minor roads that previously were largely confined to those with local knowledge. This is likely to occur when major roads are congested and represents an effective increase in the capacity of the road network, so generating additional traffic – the converse of the ‘disappearance’ of traffic when carriageway is reduced. Increased use of minor roads is problematic when policy is concerned to decarbonise the transport system and to promote active travel, which these roads facilitate.

The possible role of digital navigation might be investigated by an analysis of the correlation of the upward adjustment factor for each minor road sample location with traffic volumes on nearby major roads – to test the hypothesis that there would be more use of minor roads in areas where major roads were most congested. If so, this factor should be taken into account when setting up the new minor roads sample for the coming decade.

The use of digital navigation has been growing and may continue to grow in the future. A better understanding of the phenomenon would be important for forecasting road traffic growth by means of the National Transport Model and models at regional level and below.

A further possible cause of the changed distribution of traffic on minor roads arises from intentional interventions aimed at reducing such traffic. It has long been the practice to discourage ‘rat running’ on urban minor roads by means of suitable physical control measures, as are used in low-traffic neighbourhoods (LTN). Such measures would reduce traffic in certain locations while possibly increasing it in others through diversion. Some locations in the minor roads sample may be so affected. If LTNs and similar measures increase over time, the sample may become increasingly unrepresentative, a factor that should be taken into account in setting up the new sample. However, the net effect of intentional interventions would be to reduce traffic overall, so this cannot account for the reported growth of traffic on minor roads.

The growth of minor road use by through traffic apparently facilitated by digital navigation would strengthen the case for implementing LTN measures. Alternatively, or additionally, the providers of digital navigation might be encouraged to omit routes that direct through traffic along minor roads.

More generally, the impact of digital navigation on the functioning of the whole road network seems likely to be significant and therefore worthy of investigation.

The above considerations prompt a number of questions:

  1. How reliable are the statistics for motor vehicle use of minor roads, given the apparent sensitivity to the sampling of locations?
  2. How reliable are the NTS findings for car use?
  3. What information is available on the likely causes of the increase of traffic on minor roads?
  4. What is known of the impact of digital navigation on the road network?
  5. What are the implications of digital navigation for transport and traffic modelling?


The reported increase in motor vehicle traffic on minor roads over the past ten years is substantial and locationally heterogenous, for reasons that are unclear. This lack of understanding raises methodological questions about the sampling of minor roads. The reported increase in traffic is not consistent with the findings of the National Travel Survey, as well as being of concern to Transport for London. While interventions to reduce traffic on urban minor roads may increase the heterogeneity of the sample, they would not increase the volume of traffic. Hence this increase is most likely due to the growing use of digital navigation devices that allow minor roads to be used by those without local knowledge. This has implication for transport modelling as well as for policies to decarbonise the transport system and encourage active travel.

The coronavirus pandemic stimulated initiatives by both the UK government and local authorities to promote active travel. The rationale was that public transport would be less attractive and would have less capacity while transmission of the virus remained a problem, so that the alternatives of walking and cycling should be promoted urgently.

The Secretary of State issued statutory guidance in May 2020 to local authorities expecting them to make significant changes to their road layouts to give more space to cyclists and pedestrians. As well as a response to the pandemic, active travel was seen as affordable, delivers significant health benefits, improves well-being, mitigates congestion, improves air quality and has no carbon emissions at the point of use. Substantial government funds were provided to local authorities for this purpose.

In London, the Mayor announced a bold new plan for street space, hoping to accommodate a ten-fold increase in cycling and a four-fold increase in walking, the rationale being that with London’s public transport capacity potentially running at a fifth of pre-crisis levels, millions of journeys a day would need to be made by other means. If people were to switch only a fraction of these journeys to cars, London risked grinding to a halt, air quality would worsen, and road danger would increase. To prevent this happening, Transport for London (TfL) would rapidly repurpose London’s streets to serve this expected unprecedented demand for walking and cycling in a major new strategic shift.

I have previously commented skeptically about the feasibility of such a large shift in travel mode. But the rationale for the urgency of the government’s and the Mayor’s initiatives was to achieve a reduction in use of buses and trains. However, that reduction came about through the measures to reduce virus transmission that included encouraging all those who could work from home to do so, as well as closure of non-essential shops during the periods of lockdown. The result was that city centres were denuded of workers and shoppers, and the extra space for walking and cycling was not needed.

The was quite often local opposition to local measure to introduce Low Traffic Neighbourhoods in which car use was restricted, not least because they were introduced without consultation as matters of urgency to respond to the pandemic. In a number of cases, decisions were reversed. An important reversal arose from a recent judicial review in the High Court initiated by bodies representing London taxi drivers who complained about TfL’s decision to exclude of taxis from an important street in central London (part of the A10 route).

The judge found against the Mayor and TfL, concluding that the measures proposed in their Plan and Guidance (Streetspace for London), and implemented in the A10 order, far exceeded what was reasonably required to meet the temporary challenges created by the pandemic. However, the judge also concluded that had the Mayor and TfL proceeded more cautiously, monitoring the situation and acting upon evidence rather than conjecture, their proposals would have been proportionate to the difficulties which needed to be addressed.

So the judgement was critical of the rushed process, but does not rule out measures that change use of streets provided proper process is followed, including gathering relevant evidence, consulting with those that might be affected, and drawing rational conclusions.

The Treasury is consulting on ‘VAT and the Sharing Economy’. This is prompted by concerns for a level playing field between traditional businesses and newer models made possible by the internet, and also about loss of tax revenue.

The innovative approach of Uber, and to a lesser extent other providers using digital platforms, has made a significant improvement to the quality of transport services. The market for taxis is competitive, involving black cabs driven by owner-drivers as well as ‘minicabs’ whose owner-drivers taking bookings via a local agent. There is no evidence of tendency to monopoly by the dominant digital platform. In general, no taxis charge VAT so that providers of taxi services via digital platforms have no competitive advantage.

Accordingly, charging VAT on taxi fares collected via a digital platform would distort competition, at least while the VAT threshold applies to owner-drivers. There may be a case for abolishing the threshold for all taxi services to achieve a level playing field, although this would be onerous for those drivers who work part time to supplement earnings from their main employment.

If the VAT threshold were retained for owner-drivers other than those operating through digital platforms, the platforms might seek to preserve their competitive position by absorbing the tax through taking more commission from the drivers, although that would be limited by the need to pay enough to recruit drivers. In this situation, drivers offering taxi services via digital platforms would be disadvantaged. Alternatively, were VAT not to be absorbed by the platforms, fares would be higher to the detriment of consumers, leading to the platforms likely exiting the market, again to the detriment of consumers.

More generally, public transport fares are zero rated for VAT, so levying VAT on taxis would distort the market for non-private travel.

In short, were VAT to be levied only on fares charged by taxi services provided via digital platforms, the existence of the VAT threshold would distort competition, to the detriment of consumers.

There are suggestions, unconfirmed, that HMRC has already raised a £1.5 billion VAT assessment on Uber.

Note added 22 February 2021: The recent judgement of the Supreme Court that Uber drivers must be treated as workers, not as self-employed, may increase the likelihood that Uber would be obliged to charge VAT on fares.

I have a new paper on how time constraints affect our travel behaviour. The link to the journal is here. Some copies are free to download here. The manuscript is here. The abstract is below.

Considerable observational evidence indicates that travel time, averaged across a population, is stable at about an hour a day. This implies both an upper and a lower bound to time that can be expended on travel. The upper bound explains the self-limiting nature of road traffic congestion, as well as the difficulty experienced in attempting mitigation: the prospect of delays deters some road users, who are attracted back following interventions aimed at relieving congestion. The lower bound implies that time savings cannot be the main economic benefit of transport investment, which means that conventional transport economic appraisal is misleading. In reality, the main benefit for users is increased access to desired destinations, made possible by faster travel, which is the origin of induced traffic. Access is subject to saturation, consistent with evidence of travel demand saturation. However, access is difficult to monetise for inclusion in cost-benefit analysis. Consequential uplift in real estate values may be a more practical way of estimating access benefits, which is relevant to the possibility of capturing part of such uplift to help fund transport investment that enhances such access.

The Department for Transport is consulting on whether to extend the length of the appraisal period used to assess project benefits, typically 60 years at present. The argument is that many projects have the potential to deliver benefits well beyond this time horizon, but these benefits are not currently included in scheme appraisals. My response is set out below.

The standard approach to the appraisal of transport investments is based on the estimation of user benefits, mainly the saving of travel time. Clearly, uncertainty increases as more distant future benefits are considered. Factors that would need to be taken into account in estimating future demand include:

  • Population growth. The Total Fertility Rate (the average number of children born to a woman over her lifetime) for the UK is 1.89, below the level of 2.1 needed for a stable population, and the lowest on record. Many developed countries have lower rates: Germany 1.45, Italy 1.44, Japan 1.41. So, we may experience future population decline, although decisions on immigration would affect the outcome.
  • The relationship between income growth and travel demand. The average distance travelled by all surface modes has not increased this century (NTS data), suggesting an uncoupling of the relationship between GDP growth, income and travel demand. There is evidence for the saturation of demand for daily travel.

More generally, it is impossible to validate the performance of models far into the future. Lack of validation contributes to optimism bias in modelling generally. Models are complex and opaque, with the value of many parameters to be chosen based on expert judgement, such that outcomes can often reflect the preconceptions of those who commission the modelling. The Green Book and TAG provide uplift factors for costs to allow for optimism, but there is no equivalent for benefits.

There is evidence for optimism bias in demand estimation, particularly when competitive bidding is involved, as for rail franchises in the UK and toll road concessions in Australia. Some winning bids have been too optimistic in projecting future revenues, such that rail franchisees have withdrawn, and toll road investors disappointed and consultants successfully sued.

Given all the uncertainties, extension of the appraisal period beyond the current duration would be unwise. The possibilities of benefits beyond 60 years might be regarded as a bonus that could increase confidence in an investment that offers an acceptable BCR within that period, as well as to counter optimism bias in demand estimation.

I imagine the interest in extending the appraisal period arises from the HS2 Business Case, where extending to 100 years generates a small increase to the BCR that is otherwise in a low value for money category. Notwithstanding the arguments above, there could be a case for extending the appraisal period for HS2 on account of the expected changes in land use.

There has always been an inconsistency in supposing time savings to be the main user benefit of transport investment, given that average travel time as measured in the NTS has hardly changed over almost fifty years, despite huge investment justified by the saving of travel time. The explanation is that time savings are short run. In the long run users take advantage of faster travel to travel further, to gain more access to people, places, opportunities and choices. Increased access leads to changes in land use and in the built environment that are mainly long term.

Accordingly, it could be appropriate to appraise the long-term benefits of transport investment as part of consideration of its long-term impact on the wider built environment. In the case of HS2, this would involve assessing the prospects for business and residential property development at locations whose access is enhanced by the new rail route. That is not to say that changes in land use would continue over a long period. They may well take place fairly quickly, both before and after the rail route opens, although there would be long-term benefits from the improvements to real estate. There are many uncertainties about such developments, both planning and commercial, but these uncertainties are directly relevant to policy objectives and therefore worth addressing, unlike the uncertainties about long run travel time savings.

The underlying question concerns the nature of the long run economic benefits of transport investment. While an extensive methodology has been developed based on time savings as the main part of generalised costs, time and money are importantly different. Time acts as an independent influence on travel behaviour. The long run impact of investment is to increase access within a time constraint. Such increased access is the benefit to users and results in changes to the use and value of land made more accessible.

In conclusion, if appraisal focuses on time savings to users, then extension of the appraisal period is not justified. If, however, the focus is on increasing access to the built environment, then a longer timeframe might be warranted.

The Climate Change Committee has published a comprehensive and impressive analysis of how to achieve net zero carbon emissions by 2050. This includes a detailed treatment of surface transport, currently responsible for 22% of UK Greenhouse gas emissions, the absolute amount having changed little since 1990, stable in the range 110-120 MtCO2e annually. Cars account for 61% of surface transport emissions. Three options are proposed to secure emissions reduction:

Reducing demand for car travel by a variety of social and technological changes, including increased home working, online shopping, increased car occupancy though shared mobility, a shift to active travel and public transport, and more fuel-efficient driving.

Improving conventional vehicle fuel efficiency through regulation of road vehicle performance, use of biofuels, and more rail electrification.

Widespread deployment of electric vehicles (EVs) with the uptake of new battery EVs to reach 90-100% by 2030, in line with the government’s intention to phase out sales of new conventional cars and vans by that date. Driving range is expected to improve as battery technology advances. Sufficient charging infrastructure would be needed for the 30% of car users without access to off-street parking, as well as rapid charging for longer trips. The electricity supply system will need reinforcement.

The CCC has modelled the quantitative requirements associated with these options to show that it is possible to reduce surface transport emissions to 32 MtCO2e in 2035 and to 0.9 MtCO2e in 2050. The largest contribution comes from EVs. There are of course multiple uncertainties, many of which have been modelled.

The question is to what extent it will be possible to follow this emissions reduction pathway without measures beyond those already planned. Will it be necessary to create stronger incentives, for instance through more subsidy for EV charging facilities, or by making conventional vehicles more costly to operate through increased taxation? Changing relative prices can be a powerful incentive to change behaviour. It has not been helpful that public transport fares have risen much faster in recent years than the cost of motoring, in part due to a freeze on the rate of road fuel duty since 2010, reflecting perceived political sensitivities.

I am not optimistic about the practical possibilities that would increase the cost of car use, even for the virtuous cause of tackling climate change. We will have to make the most of regulation, the costs of which are more opaque, to effect the necessary changes.

The National Infrastructure Commission (NIC) has published its final report on Rail Needs Assessment for the Midlands and the North. This had been commissioned by the government to support its intention to prepare an Integrated Rail Plan to identify the most effective scoping, phasing and sequencing of relevant investments and how to integrate HS2, Northern Powerhouse Rail, Midlands Rail Hub and other proposed rail investments.

The projection of benefits departs from the standard transport cost-benefit analysis in which the main economic benefit to users is the saving of travel time. As set out in the annex to the report, the Commission’s approach is to assess the potential for rail investments to support both economic growth and improved quality of life, as these arise through the increase in density in city centres. Such density increase generates the well-known boost to productivity from agglomeration, an approach which is innovatively extended to capture the consumption impacts of agglomeration through access to increased amenities. The latter replaces conventional time saving benefits.

The NIC analysis recognises increased wages for workers accessing better paid jobs through increased rail capacity. Improvements in rail connectivity between cities and towns are also estimated, which contribute to increased productivity.While changes in life cycle carbon emissions and in natural capital are estimated, changes in land use are not.

The NIC’s methodology is used to compare packages of investment, formulated according to both overall cost and emphasis on enhancing links, regional and long distance. Broadly, investment in regional links comes out as a bit more attractive than in long distance links, which implies less importance to building the eastern leg of HS2 to improve journey times to London than in reinforcing connections within the regions. However, in relation to the cost of investment, none of this additional rail capacity seems very attractive, although the Commission concludes that with some assumptions about the non-monetised benefits, its analysis suggests the full benefits should meet or outweigh the costs of the packages. In this respect it is similar to the conventional economic analysis of HS2.


The NIC analysis is particularly interesting in that it is based on the recognition that travel time savings are not a satisfactory basis for estimating the benefits of transport investment, given that average travel time has not changed for at least half a century despite huge investment justified by the expectation of time savings. So the NIC focuses on the benefits of increased density of city centres that could be made possible by better rail connections. The established estimation of productivity benefits arising from higher density, the agglomeration effect, is extended to amenity benefits to consumers, a welcome innovation. Nevertheless, valuation of agglomeration effects is indirect, depending on econometric analysis, as opposed to changes in land use and value that are directly observable. So the omission of changes in land use from the NIC analysis is a pity.

The RAC Foundation has published a report on how information can be conveyed to drivers via connected vehicles. This assessed representative possibilities including In-Vehicles Signage (IVS) to display road signs and warnings to the driver inside the vehicle, and Green Light Optimal Speed Advisory (GLOSA) which tells drivers what speed to adopt to pass through the next set of traffic signals on green. The report discussed the obstacles to implementing these technologies, which it suggested arise mostly from organisational, institutional and human issues.

What was not adequately considered was the elephant in the room – the existing digital navigation (satnav) services, whether available free of charge as smartphone apps (Google Maps, Waze and others) or embedded as an integral part of the vehicle equipment. While the report mentions satnav devices as already providing some signage information, it envisages that a key element of the IVS concept is the ability for highway authorities to communicate directly with drivers, so that they can give them information they want them to receive (for example hazard warnings), as opposed to a satnav provider generating messages themselves.

Yet given the widespread use of digital navigation, it seems likely that the prime deciders of what information is conveyed to drivers will continue to be the satnav providers, not the highway authorities. The latter would need to make timely information available to the former if drivers are to benefit. But while highway authorities are subject to statutory regulation, providers of digital navigation are unregulated and are free to choose what information to provide to road users.

More generally, digital navigation has transformed how very many motorists use the road network, particularly for occasional, as opposed to regular, journeys. A choice of routes is offered at the outset of the trip, with estimated journey times, which mitigates the main perceived problem with traffic congestion – the uncertainty of time of arrival. Alternatives may be offered en route in response to the build-up of congestion. However, because the service providers of digital navigation are very secretive about their methodologies, we have little idea of the impact of their guidance on the overall functioning of the road network. For instance, we do not know if the rerouting in response to a crash on a motorway is optimal for users of the whole road network, or for drivers responding to the advice, or for neither.

There have been many anecdotal reports of digital navigation resulting in problematic use of minor roads (‘rat running’). While traffic on A and B class roads in London has been broadly stable since the mid-1990s, traffic on C roads, which had also previously been stable, increased from 5.4 billion miles in 2009 to 9.3 billion in 2019, suggestive of substantial impact of digital navigation devices.

There is clearly a case for better coordination between satnav providers and highway authorities to optimise the impact of digital navigation for all road users and to minimise environmental harms. There is in fact a legal basis for achieving this, although it has not been put into practice. The Road Traffic (Driver Licencing and Information Systems) Act 1989 requires dynamic route guidance systems that take account of traffic conditions to be licenced by the Secretary of State. This was enacted to facilitate the introduction of a pilot route guidance system that had been developed by the Transport Research Laboratory (then part of the Department for Transport), which in the event was not taken forward. A licence could include conditions concerning roads that should not be used and information to be supplied about traffic conditions.

There is a need for a review of the impact of digital navigation on the functioning of the highway system with a view to identifying ways of benefiting road users. It is very likely that exploiting digital technologies would be far more cost effective than employing costly civil engineering technologies to increase capacity.

The Department for Transport recently published a document outlining its approach to updating its Transport Analysis Guidance (TAG) ‘during uncertain times’. Two factors imply reduced travel demand: the long-term assumption about GDP growth has been reduced from 1.9% pa to 1.4%; and population growth from 0.3% to 0.15%, reflecting exit from the EU. New values for carbon emissions are also to be provided.

What was missing, I thought, was consideration of the need to update modelling, given the DfT’s intention to published a transport decarbonisation plan. Yet there are a number of shortcomings to existing modelling techniques, particularly in respect of estimating the impact of interventions aimed at reducing transport carbon emissions.

National models

The National Transport Model (NTM) is used to generate the Road Traffic Forecasts, most recently published in 2018. Scenario 7 addresses the consequences of a shift to zero emission vehicles and projects a 51% increase in road traffic 2015-2050, compared with 35% for the reference case, reflecting a reduction in fuel costs and assuming no changes to government policy on taxation.

There are, of course, sensitivities about making assumptions about future taxation. Yet mode share is influenced by levels of tax and subsidy. Arguably, both the growing proportion of SUVs and the decline in bus use have been facilitated by the freezing of road fuel duty since 2011. There is therefore a need for an approach to modelling that allows the full range of policy options to be explored, including changes in relative costs. One possibility might be to seek a remit analogous to that given by HMT to the National Infrastructure Commission, which must be able to demonstrate that its recommendations are consistent with gross public investment in infrastructure of between 1.0% and 1.2% of GDP in each year between 2020 and 2050.

Taxation aside, the growth of traffic projected in Scenario 7 is implausible. Travel time has been measured in the National Travel Survey (NTS) for the past 45 years and on average has remained close to an hour a day. This implies that the time available for travel is constrained. A reduction in fuel costs therefore would not lead straightforwardly to an increase in distance travelled, which would only arise if either higher speeds were possible (not to be expected from a switch to zero emission technology) or higher car ownership occurred (not assumed in the model).

More generally, the three key parameters of the NTS – average travel time, trip rate and distanced travelled per year – have not increased since 2000. I would expect any model to hold these per capita parameters unchanged on a central case projection, unless there were to be a clear causal explanation for a different trend. Population growth is then the main determinant of future traffic growth, but the relative mode share would depend on where the additional inhabitants live: to the extent they are housed on greenfield sites, car use would be important; to the extent they are located within existing urban areas, investment to support active travel and public transport would be relevant. My understanding is that the National Trip End Model (NTEM) provides a single national set of assumptions about demographic factors, and therefore does not allow consideration of policy options in respect of spatial location. If public transport and active travel are to be ‘the natural first choice for daily activities’, then the spatial location implications of population growth need to be incorporated into modelling.

Regional models

Beneath the NTM, there are a number of regional transport models. Those commissioned by Highways England are mainly (entirely?) based on the SATURN software first developed in 1980. Despite very considerable ex ante efforts to refine and update such models, there is a dearth of ex post analysis of modelling validity. A partial exception is the detailed monitoring of traffic for each of the three years after opening of the widened M25 between J23 and J27. Small time savings were found at year one, but these were lost by year two due to increased traffic volumes. The forecast traffic volumes derived from the model were less than observed and the forecast increase in traffic speed did not materialise, hence negating the economic case for the investment. The additional traffic generated externalities beyond forecast, including carbon emissions.

More generally, the whole area of regional modelling lacks transparency. Highways England does not appear to publish information on its models and their validation. It would be timely to review the validity of such models.


Current UK transport modelling as a whole seems mainly concerned to update, refine and apply long established approaches. The bulk of modelling expertise is found within the consultancies, who are concerned to meet the needs of their clients using accepted methodologies, often to provide formal justification for a preferred investment. The Department is conservative in its requirements. Consultants therefore have little incentive to develop innovative approaches. Fresh thinking is needed, yet there is no academic centre of expertise in transport modelling where innovation could be expected.

Established models do not seem well suited to supporting a strategy aimed at achieving net zero transport carbon emissions by 2050. A related problem is the lack of data for model calibration in respect of the impact of the range of possible policy interventions. For instance, if the encouragement of active travel is successful, from which mode does the shift occur? The experience of the cycling city of Copenhagen is that car use is only slightly less than in London, but public transport mode share is half that of London. This suggest that we can get people off the buses onto bikes, but that it is more difficult to get them out of their cars, even in a city where all motorists are familiar with cycling.

Decarbonisation will be a long game, during which we should be able to gain understanding of the consequences of the various policy interventions, even though these will be difficult to model at the outset. It would be desirable to initiate the development of new models soon, ready for when calibration data becomes available.

The main focus of recent interest in the area of ‘Connected and Autonomous Vehicles’ has been autonomy – driverless cars – where both tech companies and auto manufacturers are attempting to get the technology to the point where it could be used on real roads. In contrast, while the idea of connected vehicles has been around for some years, progress has been slow. There are two kinds of connectedness: vehicle-to-vehicle (V2V) and vehicle-to-infrastructure (V2I); collectively V2X. A recent US webinar and report provided some illumination.

The main motivation of V2X is to improve safety, a real issue in the US where more than 6000 pedestrians a year are killed in traffic-related incidents. The aim is to enhance visual line of sight communication by using other parts of the electromagnetic spectrum.

There are two means of connectivity: WiFi based on a dedicated short-range communications (DSRC) standard approved in 2010 but with little implementation by manufacturers; and a cellular connection (C-V2X), based on smartphone technology, which could connect vehicles directly, independent of the base stations used for normal voice calls. Initially, 4G technology is being used, with the intention to move to the much faster 5G as that is rolled out. The hope is that vehicles will also be able to sense the presence of pedestrians by detecting their smartphones. The V2I capability depends on road authorities being willing to install connectivity in traffic signals and other roadside signs, which they may be reluctant to afford.

Deployment of V2X depends on whether this is mandated by national authorities. The Chinse government is supporting C-V2X. On the other hand, EU states voted against a proposal of the European Commission to adopt a WiFi standard, and the US government has also not supported a similar approach. In the absence of a policy mandate, deployment depends on consumer demand. The VW Golf launched in 2019 has WiFi V2X connectivity. Ford is planning to introduce C-V2X in 2022.

However, consumer interest and willingness to pay for a range of driver assistance and connectivity technologies has remained lukewarm for several years, with substantial levels of ambivalence and even scepticism towards these offerings. Enhanced safety, while desirable, may be insufficient a selling point. And having two technologies in use will not help.

Apart from safety, the other potential use of V2X connectedness would be to increase effective road capacity by permitting shorter headways between autonomous vehicles. This benefit depends on the implementation of robot-driven vehicles with faster reaction times than human drivers. Shorter headways would increase the risk of crashes. Crashes involving autonomous vehicles require allocating responsibilities for fault, more difficult where V2V communication is involved between vehicles of different make under different ownership. Moreover, the benefit from increased road capacity accrues to road authorities more than to road users, so the commercial incentive to develop V2V for this purpose seems quite limited.

Altogether, the case for developing vehicles connectedness does not seem strong.