Select Committee on Transport, Local Government and the Regions Appendices to the Minutes of Evidence

Memorandum by the Institute for Transport Studies, University of Leeds (RTS 39)



  This memorandum, written in response to the invitation by the Committee to submit material for the inquiry on Road Traffic Speed, is focussed on how new technology in the form of Intelligent Speed Adaptation (ISA) can assist in tackling the problem of excess and inappropriate speeds. Over the last few years, the technologies and systems that can deliver ISA have matured to the point where ISA is now a completely practical proposition.


  Intelligent Speed Adaptation (ISA) is a system by which the vehicle "knows" the permitted or recommended maximum speed for a road. The standard system uses an in-vehicle digital road map onto which speed limits have been coded, combined with a positioning system which could be GPS, ie the satellite Global Positioning System, but could also be GPS enhanced with map matching and dead reckoning. ISA can take various forms:

    —  In terms of intervention level, it can be advisory (the driver is informed of the limit and of violations), voluntary (the system is linked to the vehicle controls but the driver can choose when to have the system enabled), or mandatory (no override is possible).

    —  The speed limit information can potentially be extended to incorporate lower speeds at certain locations in the network and even in the future variation with current network conditions, based on weather, traffic density, the presence of incidents etc.

  In addition to research in the UK, ISA trials have taken place in Tilburg in the Netherlands and Aalborg in Denmark. Large-scale trials are currently almost complete in four locations in Sweden.


  An ISA system can be characterised by how intervening (or permissive) it is. Here the standard variants are:

  1.  Advisory—display the speed limit and remind the driver of changes in the speed limit;

  2.  Voluntary ("Driver-Select")—allow the driver to enable and disable control by the vehicle of maximum speed;

  3.  Mandatory—the vehicle is limited at all times.

  An additional possible variant between (2) and (3) is a mandatory system which allows excursions allowed, eg for overtaking. Such excursions could be limited in number per unit of time or frequency per length of road.

  Another dimension for differentiating ISA systems is that of the currency of the speed limits themselves. Here the major typology is:

  1.  Fixed—the vehicle is informed of the posted speed limits (those displayed on static signs by the side of the road);

  2.  Variable—the vehicle is additionally informed of certain locations in the network where a lower speed limit is implemented. Examples could include around pedestrian crossings or the approach to sharp horizontal curves. With a Variable system, the speed limits are current spatially.

  3.  Dynamic—additional lower speed limits are implemented because of network or weather conditions, to slow traffic in fog, on slippery roads, around major incidents, outside a school at drop-off and pick-up times, etc. With a Dynamic system, speed limits are current temporally.


  This project, funded by DETR as it then was, ran from 1997 to early 2000. The partners in the project were the University of Leeds and the Motor Industry Research Association (MIRA), and the project had a wide-ranging brief to look at almost every aspect of ISA, ranging from suitable technologies, through public attitudes and willingness to pay, to side effects in terms of travel time and fuel consumption, and finally to accident savings and implementation aspects. A major aspect of the project work was a set of user trials both on real roads (where driver behaviour could be studied in a naturalistic setting) and on a driving simulator (where complete control over the conditions experienced by the drivers could be assured).


  This study required volunteer drivers to drive a predetermined route in a car equipped with an ISA system. A Ford Escort was equipped with two versions of ISA, Driver Select and Mandatory. The software compared the appropriate speed limit with the car's actual speed. If the car was travelling below the speed limit, it behaved as a normal car. However, if the driver attempted to exceed the speed limit, engine management and, if required, mild braking were used to curtail vehicle speed.

  The test route was selected to include roads of varying speed limits and classes, and was approximately 42 miles in length. Speed limits varied from 30 to 70 mph and included urban roads with mixed traffic and a high number of pedestrians, rural roads and a motorway section. In total there were 18 speed changes on the test route. The cars were instrumented for automatic data collection. Behavioural observations were made by two in-car observers with regards to driving errors, interaction with other drivers and conflicts. Workload and acceptability were assessed using questionnaires. There were 24 participants in the experiment. They drove on three separate occasions; 16 drove the car once without an ISA system and the following two times with one version of the system (eight with the Driver Select version and eight with the Mandatory one). The other eight participants drove all three times with the system off, to provide a baseline reference group. Thus there were 72 drives in all.

  With regard to the Driver Select system, drivers were generally happy to leave the system engaged, but as soon as the opportunity to exceed the speed limit arose, they chose to disengage the system. Thus, it was exactly in the locations that the system would have had the most impact, ie the rural villages and urban roads where traffic generally exceeds the speed limit, that drivers were more inclined to switch the system off in order to break the speed limit. It was also noted that drivers used the system less on their second drive, indicating there may be shifts in behaviour depending on the amount of exposure to the system.

  The Mandatory system, as would be expected, had a far greater impact on driver behaviour. Large reductions in maximum speeds were noted on most road sections, especially in urban areas and rural villages, as can be seen from Figure 1.

  The data shows quite clearly that, in the absence of the ISA system, drivers are poor at adapting to low speeds after travelling through a higher speed limit area. ISA was effective in preventing speeding. There was no change in speed distribution at the lower end, indicating that drivers were not "driving on the system", something that opponents of ISA have predicted as an effect. Instead, they were properly responding to traffic conditions around them.

  The in-car observers counted conflicts, ie critical interactions with other road users. With ISA, performance improved as compared with the initial run without ISA. In addition those subjects who drove every time without ISA tended to have more conflicts with increasing familiarity with the route. From the questionnaires, drivers with the Mandatory system felt they paid more attention to the driving task, and as a result were more aware of upcoming hazards. They also reported they felt they had more time to make decisions due to their lowered speed.

  In terms of driver acceptance, the Driver Select system was thought to be more useful than the Mandatory system. Generally, driver opinion about the systems changed little over the course of the trials. The interviews revealed that drivers regarded the Driver Select system as more of a safety system than the Mandatory, and less of a source of frustration. So the objective data indicate better performance with the Mandatory system, but there was a clear preference for the Driver Select version.


  The aim here was to evaluate behaviour with a variety of ISA systems. The simulated road included urban, rural and motorway sections, providing a range of speed limits between 30 and 70 mph. Other cars in the scene provided the opportunity to study overtaking scenarios, gap acceptance tasks and car-following situations. The road environment also featured traffic lights and pelican crossings in order to instigate possible violations; and sub-standard curves were included in both the urban and rural sections. Workload and acceptability were also monitored.

  The results suggest that ISA had little impact on mean speeds, but reduced maximum speeds. The effects of ISA on speed were most prominent at specific locations, such as village entry. Several additional changes in behaviour were identified. It was found that, when using ISA, gap acceptance behaviour at junctions altered. The mean gaps accepted and the minimum times to collision reduced in size, suggesting that drivers were exhibiting somewhat riskier behaviour. There were also observed changes in car following behaviour. When driving behind a slow moving vehicle (with no opportunity to overtake), drivers using ISA were more likely to want to engage in close following on both urban and rural roads.

  Subjective mental workload scores were obtained and, both time pressure and frustration increased as drivers used the system more and more. This time pressure does not translate into actual loss of time, as there was little change in total journey time for each of the progressive runs. Thus this increased time pressure is only imaginary, not actual, but it does serve to explain the gap acceptance and close flowing results.

  The results confirm the potential benefits of ISA, with reduced maximum speeds and improved speed adaptation in speed limit transition zones. But there were also some secondary effects of using ISA. These effects are unlikely to outweigh the substantial safety benefits of reduced maximum speed and reduced variability in speed, and they may just be short-term effects resulting from not being accustomed to using ISA. Only data from long-term trials can show whether such effects persist.


  Table 1 shows the best estimates of the accident savings, at various levels of accident severity, for the permutations of ISA. ISA systems are divided into the broad classes of Advisory, Driver Select, and Mandatory systems. Each broad class can have speed limits in fixed, variable or dynamic forms (where dynamic also includes variable capability). The predictions are based on the premise that all vehicles could be fitted with ISA instantaneously. The most powerful and versatile form of ISA, the Mandatory Dynamic system, is predicted to reduce overall injury accidents by 36 per cent, fatal and serious accidents by 48 per cent and fatal accidents by 59 per cent. Because there is a particularly strong relationship between collision speed and the probability of injury, of serious injury or death in accidents involving pedestrians and cyclists, vulnerable road users would be among the major beneficiaries from ISA.


Speed Limit
Predicted Injury
Accident Reduction
Predicted Fatal and
Serious Accident
Predicted Fatal
Accident Reduction
Driver Select

  The predicted savings with Mandatory Dynamic ISA are considerably greater than was achieved with perhaps the most effective single measure to date in Britain, the introduction of compulsory seatbelt wearing for front occupants, which achieved a 20 per cent reduction in fatalities to front seat occupants, ie a 7 per cent reduction in fatalities overall (Harvey and Durbin, 1986). It is perhaps worth noting that this prediction of the safety potential of speed limiters for Britain is almost exactly in line with the prediction of a 30-40 per cent reduction in injury accidents for Sweden from Va«rhelyi (1996).


  The External Vehicle Speed Control project recommended that the Mandatory Dynamic system be the ultimate target. But even if the goal is to move towards only a voluntary system, the requirements and timetable are virtually the same. Figure 2 shows the major prerequisites and stages to implementing ISA. The stages and decision points are:

Further research
Decision to move forward towards full implementation
Preparation and enactment of standards
Promulgation of standards
Preparations for production on new vehicles
Mandatory fitment on new vehicles [62]
Voluntary usage
Potential requirement for mandatory usage

  In many ways this timetable is disappointing, because the benefits are postponed so far into the future. Earlier work on standards and other enabling technologies, such as the preparation of an authorised national digital road map incorporating speed limits would assist here, not least in promoting voluntary take-up particularly by fleets.

4.5  Cost-benefit analysis

  A cost-benefit analysis was carried out using the standard UK procedure. The analysis took into account:

    —  The predicted accident savings.

    —  Estimates of fuel savings obtained by modelling of various road networks. This work indicated that, depending on road category, overall fuel consumption savings with ISA would range from 1 per cent to 8 per cent, with an overall figure of 5.5 per cent.

    —  The estimated costs at various dates of the equipment required on the vehicle to provide ISA. For the dynamic systems, costs of sensors and broadcast were also computed, and for all types of ISA the costs of maintaining a national database of speed limits was included.

    —  The transition time to ISA implementation, including the gradual rollout of ISA vehicles into the national fleet.

  The results are shown in Table 2. Clearly the Dynamic Mandatory variant provides the most attractive solution, with a benefit-to-cost ratio of 12.2 for the low GDP growth scenario, and 16.7 for the high GDP growth scenario.

Table 2


Low GDP Growth
High GDP Growth
Driver Select

  All the benefit-to-cost ratios are in excess of 3.5, ie they would indicate a positive decision to move ahead under standard investment guidelines. Mandatory ISA has considerably higher benefit-cost ratios than the Advisory or Driver Select systems. The largest ratios are for the Mandatory Dynamic system: 12.2 for the low GDP growth scenario, and 16.7 for the high GDP growth scenario. This is therefore the system of choice.


  The new UK ISA project began in January 2001 and has a duration of 52 months, which may be extended by six months. The project partners are the University of Leeds and the Motor Industry Research Association (MIRA). The main tasks of the project are:

    —  To investigate user behaviour with ISA by means of a set of field trials;

    —  To study overtaking behaviour with ISA in a driving simulator;

    —  To prepare an ISA design for motorcycles and large trucks and to build a demonstrator of each;

    —  To prepare a system architecture for a mass production configuration of ISA;

    —  To have an input into relevant standards activities at an international level;

    —  To carry out a process of technology watch throughout the project duration;

    —  To further investigate the costs and benefits of ISA.

  The field trials are the major focus of the project and their overall aim is to study long-term behaviour with and acceptance of ISA. It is planned to equip 20 vehicles with ISA and data collection capability for the field trials. The field trials will start in 2002 and will each last for six months. Four successive trials are planned:

  1.  West Yorkshire, private motorists

  2.  West Yorkshire, fleet

  3.  Midlands, private motorists

  4.  Midlands, fleet

  The West Yorkshire area will be centred on Leeds and thus will be mainly urban; the Midlands area will be mainly rural and small town. The total period for the four trials will be 30 months (there is an allowance of two months between each individual trial for the refurbishment of the vehicles). At the moment, the project's timetable envisages starting the first trial in October 2002. The last trial would therefore end around March 2006.

  The trials are designed to be non-intrusive—the vehicles will look and behave like "normal" cars apart from the ISA feature, data will be logged automatically, and data collection will be remote through a digital mobile phone link. The ISA system to be fitted will be of the voluntary type: drivers will be able to opt out of ISA speed limiting at will via a button on the steering wheel, although the default situation will be that ISA is on. There will also be a kickdown feature so that sharp pressure on the accelerator will result in the ISA going into standby, with the ISA resuming only when the driver voluntarily comes below the speed limit. The speed limit and ISA state will be indicated to the driver by a screen which will be integrated into the dashboard. It is intended to provide drivers with ISA support for a large percentage of their regular driving, to include both the local area and the national trunk road network.

  The first year of the project has been spent on designing and fitting a prototype vehicle, which is to be used as the basis for the fleet of 20. This vehicle has been completed and is available for demonstration through requests to DTLR.


  ISA is more than justified in cost-benefit terms, but its main benefits are a long time in the future and there is a real danger that the benefits will be even further postponed through inaction and anxiety over the political fallout. It is my firm belief that, once the debate is out there in the open, there will be firm public support for moving ahead. There is no need for a decision now on whether the UK should go with mandatory ISA, but there is a need now to move ahead so as to enable ISA to be realisable in some form. Concrete actions that should be taken are:

  1.  Creating a national database of UK speed limits and procedures for updating that database.

  2.  Assistance in developing standards, particularly on how speed limits should be encoded into digital road maps.[63]

  3.  Encouraging voluntary take-up of ISA, particularly by fleets.

  4.  Research on how to take forward dynamic ISA (in terms of sensors, decision rules, procedures and communications).

  5.  Moving towards a European requirement for ISA capability on all new vehicles sold from around 2013.


  Harvey, A C and Durbin, J (1986). The effects of seat belt legislation on British road casualties: a case study in structural time series modelling. Journal of the Royal Statistical Society, A149, 187-227.

  Va«rhelyi, A (1996). Dynamic speed adaptation based on information technology: a theoretical background. Bulletin 142. Department of Traffic Planning and Engineering, University of Lund.

Dr Oliver Carslen

Director of Research

January 2002

62   It is envisaged that the requirement for fitment of ISA capability would be enacted at a European level. It would be left to the Member States to decide how to use that capability-the national requirement could be for ISA to be used in advisory mode, it could be for voluntary (Driver Select) ISA, or eventually it could be for mandatory ISA. Back

63   There is already an internationally agreed exchange format for digital road maps, known as GDF3. Speed limits could easily be accommodated within that specification. Back

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