Speed limiters and intelligent speed adaptors

Speed limiters limit the maximum speed of a vehicle. If compulsorily equipped on all vehicles, these systems have a great safety potential, due to lower, more homogenous speeds and thus fewer overtakings. However, if the speed limiter is to give significant safety improvements it must work automatically and be compulsory. Speed limiters in vehicles restrict the engine’s fuel injection when the vehicle reaches a certain speed. Such devices are already installed on certain types of vehicles as mopeds, tractors and, in some European countries, on heavy vehicles and buses. Hydén and Almqvist (1987) detail the different technical devices. Várhelyi (1996) analysed the occurrence of accidents under differing road and lighting conditions and concluded that a speed limiting system would lower the number of police reported injury- accidents by between 19% and 34%. The author also states that this is a conservative estimation, because it does not include accidents that occur under impaired visibility such as those in fog or on sharp curves. It is suggested that a more optimistic estimate is a reduction in accidents between 24% and 42%.

The first trial with a speed limiter in passenger cars was carried out in France (Saad and Malaterre, 1982). The driver could set a speed limit, which could not be exceeded unless the driver disengaged the equipment by a separate operation. The test drivers adapted their speed in relation to the traffic situation around them, giving rise to small changes in speed around the speed limit. In order to “float with” the other cars, drivers had to disengage the speed-limiter quite frequently which was reported to be physically tiring. They found keeping to the speed limit on 60 km/h and 80 km/h roads difficult. On 90 km/h roads the use of limiter depended on the traffic volume, such that a higher traffic volume resulted in frequent changes in speed and thus less use of the limiter. On roads with speed limit of 110 km/h and 130 km/h where monotonous driving was more common, the limiter was used more frequently. Most drivers set the top speed on the limiter significantly above the speed limit and it was found that as the speed limit on a road decreased, the difference between the speed set on the limiter and the road speed limit increased. On shorter stretches of road with a lower speed limit, e.g. through villages or curves, the drivers did not adjust the speed set on the limiter. In addition, the knowledge that the speed limited vehicle had reduced acceleration availability, meant that drivers in some circumstances chose not to perform an overtaking manouevre. The test drivers also thought that the speed control was too effective and that consequently their freedom was limited.

A field study in Sweden (Persson, Towliat, Almqvist, Risser and Magdeburg, 1993) showed that mean speed decreased on links by between 2% and 8% with the speed limiter and there was a slight tendency to compensate the low speeds on stretches by driving faster (by 2-3 km/h) through the junctions (perhaps an unconscious compensating behaviour). Data from behaviour observations showed a clear increase of the proportion of correctly kept distance to the car ahead (even on sites with speeds lower than the actual speed limit). On the other hand, there was a slight increase of incorrect behaviour against other road-users at junctions. Most of the drivers generally displayed positive attitudes towards the speed limiter regarding general impression, but did sometimes experience feelings of impatience. 85% of all drivers reported positive attitudes with regards to feelings of safety. The most often mentioned advantage for drivers was smother rhythm in traffic and better preparedness for unprotected road-users. The

most commonly mentioned disadvantage for drivers was that it would be impossible to accelerate and sometimes exceed speed limits.

Behavioural adaptation at junctions was also demonstrated in a simulator study (Comte, 1996).

An experiment carried out on the University of Leeds Advanced Driving Simulator evaluated the effects of speed limiters on driver behaviour. Safety was measured in terms of following behaviour, gap acceptance, traffic violations, and subjective mental workload was recorded using the NASA-RTLX. It was found that although safety benefits were observed in terms of lower speeds, longer headways and fewer traffic light violations, drivers compensated for loss of time by exhibiting riskier gap acceptance behaviour and delayed braking behaviour. When speed limited, drivers’ self-reports indicated that their driving performance improved and less physical effort was required, but that they also experienced increases in feelings of frustration and time pressure.

A Finnish traffic simulation study on the effects of compulsory speed limiters on heavy vehicles showed that traffic safety would improve due to a decrease in accidents (Kulmala and Beilinson, 1993). The decreases in travel speeds were larger for heavy vehicles (at the highest between 2.2 and 2.9 km/h at free flow) than for cars. The effects on the standard deviation of speeds were quite small. Gynnerstedt, Risser and Gutowski (1996) also studied the effects of a speed limiter in cars by a simulation model on two-lane rural roads at different traffic volumes in Sweden. All vehicles were assumed to be equipped with a speed limiter set at the current speed limit. The results showed that the mean speeds were unchanged, while the standard deviation decreased. The overall number of overtakings decreased, however the number of critical overtakings increased especially for high traffic volumes.

Davidsson (1995) simulated a fully implemented dynamic speed adaptation system in a central built-up area in Sweden. The maximum allowed speed through junctions was limited to 25 km/h. The speed variation decreased from 8.4 km/h to 4.2 km/h, while the mean speeds, in spite of lower maximal speeds, increased slightly. This is due to the fact that the vehicles drove in platoons in a larger extent and passed the junctions smoother. It was concluded that the system would give travel time savings up to 25%.

In an ongoing project in the Swedish town of Eslöv a number of company cars are being equipped with speed limiters. On all entry roads transponders (radio transmitters) are mounted on the 50 km/h speed signs. When the equipped cars pass the transponders the speed limiter is automatically set on the 50 km/h speed limit and when the cars leave the town the speed limiter is deactivated. Results of the evaluation of the trial (observations of driver behaviour, speed profiles and interviews) are expected to be available at the end of 1997.

In July 1996, the Ministry of Transport, Public Works and Water Management in The Netherlands, presented a new multilayer programme for road safety including a pilot study to assess the feasibility of using intelligent speed adaptors (ISA). The project will be carried out on a new housing estate in Tilburg to be built in 1998, the main aim of which will be to increase public acceptance and support for such a device. The study hopes to deal with aspects such as technical feasibilty, integration, legal framework and organisational consequences. Three technological variants have been chosen for detailed examination :

• direct adjustments to the engine to make it impossible to drive faster than the local speed limit.

• increased counterpressure on the accelerator pedal when the speed limit is exceeded

• an auditory signal when the speed limit is exceeded.

A preliminary report (Ministry of Transport, Public Works and Water Management, 1997) recommends that both the compulsory ISA variant and the intelligent speed information variant (informing the driver when the local speed limit is being violated) could be operational in 1998;

the former of these will be intended application. It is suggested that the intended structure of the new housing estate is in principle suitable for the integration of ISA and it will be possible to promote road safety by making a planned long straight road only accessible for ISA cars.

Around 600 leased cars will be fitted with ISA and road signs will be installed with beacons.

Perrett and Stevens (1996) suggest the key costs and benefits from automatic speed control in the U.K. would be as described in table b. They go on to suggest likely accident savings if such an implementation were to be introduced by using a test scenario based on mandatory automatic speed control implemented on all roads in Great Britain. The envisaged system would consist of a transceiver linked to an in-vehicle display and speed governer, which is operated by the engine management system. They postulate that speed related accidents would have to be reduced by 50% in order for the benefits to match the costs. However, some of their assumptions, e.g. on the number of relevant accidents and on infrastructure costs, are questionable. In addition, they did not conduct any sensitivity analysis.

Table B. Key costs and benefits from automatic speed control

Key costs and benefits Justification Public investment costs for

transponders: £145.8m

Hypothesis that speed limits change every 5 km Public investment cost:

£250/new car

Operating cost: 1% of investment cost/year

Hypothesis based on cost of existing systems

Accidents reduced by 16500/year Hypothesis based on PROMETHEUS research which suggested that 16% of accidents would be avoided, and that half of the potential would be realised by this

application, due to the lack of facility to vary speed limits dynamically

Fuel use on motorway journeys reduced by 89.5m litres/year

Estimate based on data on fuel consumption at different speeds, vehicle speed survey, and distance travelled on motorways

Enforcement costs reduced by

£50m/year

Hypothesis that a nominal saving is made when implementation reaches 90% of vehicles

Infrastructure investment cost reduced by £25m/year

Hypothesis that a nominal saving is made when implementation reaches 90% of vehicles

Several studies with different systems for speed adaptation, based on new technology, show that the acceptance of such systems is fairly good. The SARTRE study (Dahlstedt, 1994) showed that devices to control the speed of cars “when the driver is free to turn it on or off”

were favoured by about 46% of the respondents, while about 42% were against. According to Nilsson (1996) it is reasonable to assume that systems which increase safety and efficiency, are reliable and user-friendly and are not used for control of the driver will be gradually accepted by the drivers.

4 CONCLUSIONS

This report has summarised the research findings regarding various methods of influencing driver speed. Results of these studies are conflicting probably due to the wide variety of road types, traffic conditions and evaluation methods employed. The following section attempts to assess the various implementations in order to draw conclusions as to which are worthy of further investigation in the planned simulator trials.

Engineering the road and its immediate environment have been shown to have long-term effects on changing driver behaviour (Russam, 1979; Silcock and Walker, 1982; Wright and Boyle, 1987). The effects can be lasting in time, however their effect in space mostly ceases outside the vicinity of the measures and driver acceptance is not always high. The most efficient speed-reducing physical measure is the road hump with stable, long term local effects.

However, road humps can cause problems and inconvenience primarily for emergency services and bus operators and can produce noise and vibration, seen as detrimental to both road user and local residents. ‘Platforms’ or ‘tables’ seem more acceptable to drivers than ‘standard’

humps and can be adequately effective in speed reduction.

Diminishing lines and rumble strips have a good initial effect but this can be short lived.

Rumble areas and strips are suitable where accident reports appear to show that drivers were unaware of potential hazards which may be linked to the speeds of the vehicles. Rumble strips should be well signed to alert drivers to their presence and thus prevent excessive braking. In residential and urban areas noise and vibration are sensitive issues and the usefulness of rumble devices may thus be limited. The most suitable locations for these devices are thought to be at junctions, sharp bends, gradients and other hazards identified on rural roads which have accident problems but do not justify major reconstruction on economic grounds.

Isolated measures, such as advisory speed limits, warning signs, gateways, road narrowings and chicanes have only a small effect on speeds. Chicanes can work well at slowing traffic but their effectiveness is related to the severity of the geometry and care must be taken when access is required for larger vehicles or on bus routes. ‘Throttles’ or narrowings can be effective, but really only when two way vehicle flows are relatively high so that greater caution is required by drivers. In addition, they may reward aggressive driver behaviour and require adequate signing and marking in advance to avoid becoming a safety problem themselves.

‘Gateway’ treatments tend to be ineffective unless they incorporate some form of physical narrowing including vertical design elements or chicanes. The effect of gateways can be short lived and repeater features are required to maintain speeds at a lower level. It appears that by breaking up the perceived ‘straightness’ of a road driver speed can be reduced, thus the introduction of a mini-roundabout or changed junction priorities can have a calming effect particularly when linked to other features. The evidence on the effectiveness of curve warning signs and advisory speed signs is contradictory in terms of their effect on speed, but it appears that signs should justify their presence by specifying the possible hazard or other reason for the required lower speed. Delineation systems are roadway markers that provide the driver with information about the path and demands of the road. However several studies have indicated

that negative safety benefits may accrue as they could encourage increases in speed due to overconfidence on the part of the driver and improved visibility.

Combinations of traditional measures often work well together where individually they may have little effect (e.g. Pyne, Dougherty, Carsten and Tight, 1995). Traffic calming measures should be implemented as part of an overall Urban Safety Management Strategy as they need to be viewed in terms of the whole transportation system. It is also possible that the installation of traffic calming measures on isolated roads causes local diversions onto other roads and thus create accident migration. Traffic calming may have the potential to stem the flow of traffic in local environments, but if the arterial and collector roads are incapable of carrying the excess traffic, there will be no overall gain to the system. Area-wide measures and environmentally adapted through-roads are effective within the implementation area, but compensating effects can arise in the form of driving faster outside these areas.

Research investigating the effects of ATT speed reducing measures has produced mixed results. Variable message signs have overall produced small reductions in speed, and any effects that have been found have generally dissipated very quickly. The literature surveyed appears to suggest that VMS signs need to be relevant to the driving situation, otherwise drivers tend not to be influenced. For example, some success was achieved using VMS signs in foggy conditions, i.e. the actual fog served as the relevant factor. Despite the frequent lack of observed success, the effects of VMS, particularly when displaying speed recommendations, are increased where brief text giving a reason adds to the sign’s credibility. The language and standardisation problem deserves attention in the case of signs which are predominantly text based, especially because pan-European travel is expanding. In a limited number of cases the use of pictograms could help to convey the messages. Similar conclusions can be drawn for ATT advisory signs and variable speed limits. “The speed reducing capacity of variable speed limits appeared to be largest if the reason for the current limit is indicated in the form of warning or advice” (Tenkink, 1988 in Rooijers et. al., 1992).

The ability to provide feedback directly to the driver has shed some light on driver’s motivations. The effectiveness of providing feedback to the driver may not be due to the feedback per se, but in its implication that enforcement officials are monitoring the motorist’s speed. This hint of implied enforcement may be critical to the success of speed feedback mechanisms. It also seem important to time the feedback appropriately; immediate feedback appears to be the most effective.

In-car equipment, such as active gas pedals and ACC systems have a good safety potential if they are intervening, i.e they automate part of the driver’s task. However, driver acceptance (before driving with such equipment) is higher for those with informative functions that simply provide advice. However, it has been found that driver acceptance usually increases after having driven a vehicle equipped with such equipment. Systems that actually manipulate the controls of the car, e.g. ACC and speed limiters could possible give rise to behavioural adaptation in various forms. ACC is not a collision avoidance system, since in emergency situations the driver himself has to intervene and brake (Nilsson, 1996). It is in these situations that safety may be compromised: automation may reduce driver workload, but if inattention also arises then in critical situations, driver’s reactions may be less efficient. ACC has

limitations: it cannot detect stationary objects, its braking capacity is limited and the driver has to intervene, and it works only within a certain speed interval. Perrett and Stevens (1996) suggest that AICC is likely to be developed as an autonomous in-vehicle system to be supplied by car manufacturers. Approximately 22 prototype AICC vehicles have been produced by European motor manufacturers, but have not been technically perfected, in terms of sensors.

Speed limiters in vehicles can efficiently control compliance with the speed limit, but there is a tendency to compensate for low speeds on stretches by driving faster in situations when the appropriate speed is lower than the current speed limit.

As well as considering safety benefits and costs of the systems discussed here, it is also necessary to establish the concepts of distraction, liability and acceptability (both by car manufacturers and the general public). Overall, it appears that providing relevant, timely information to the driver is of paramount importance, and that the effectiveness of this information is enhanced if there is intimation of enforcement. If however, more intervening measures are required, a system such as a speed limiter, or intelligent speed adaptor, seems to offer a global solution. It is necessary however to discover the most reliable and acceptable solution that limits negative effects of behavioural adaptation.

The aim of this paper was to consider the safety contribution of ATT and non-ATT speed reduction measures in order to evaluate the most promising on a driving simulator. At the time of writing it is considered that perceptual countermeasures, such as road narrowing, are worthy of further investigation and will be compared to ATT systems such as speed limiters and feedback variable message signs.