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Intelligent Speed Adaptation (ISA)

Intelligent Speed Adaptation (ISA)


Intelligent Speed Adaptation (ISA)

Intelligent Speed Adaptation (ISA) is an in-vehicle system that supports drivers' compliance with the speed limit. ISA is in fact a collective term for various different systems. Field trials and driving simulator studies show positive effects on speed behaviour and expect large safety effects. Some studies report negative side effects of ISA, but there is yet insufficient insight in the size of these possible negative side effects and their consequences. Around one quarter of European car drivers considers a speed-limiting device like ISA to be very useful; actual experience with ISA seems to increase acceptance.

Field trials of ISA

Practical experiments with ISA have been carried out in Sweden and the Netherlands. In Sweden it concerned large-scale experiments with the open ISA (warning/informing) and the half-open ISA (the active accelerator) in four different town/cities. In the Dutch town of Tilburg, there was an experiment with the closed ISA. The experiments in both countries concerned urban areas. Other field trials have taken place in Belgium, Denmark, Britain, Finland, Germany, France, Hungary and Spain (see for more details).

What is ISA?

Intelligent Speed Adaptation (ISA) is an in-vehicle system that uses information on the position of the vehicle in a network in relation to the speed limit in force at that particular location. ISA can support drivers in helping them to comply with the speed limit everywhere in the network. This is an important advantage in comparison to the speed limiters for heavy good vehicles and coaches, which only limit the maximum speed.

ISA is a collective term for various systems:

  • The open ISA warns the driver (visibly and/or audibly) that the speed limit is being exceeded. The driver him/herself decides whether or not to slow down. This is an informative or advisory system.
  • The half-open ISA increases the pressure on the accelerator pedal when the speed limit is exceeded (the 'active accelerator'). Maintaining the same speed is possible, but less comfortable because of the counter pressure.
  • The closed ISA limits the speed automatically if the speed limit is exceeded. It is possible to make this system mandatory or voluntary. In the latter case, drivers may choose to switch the system on or off.

The currently available ISA systems are based on fixed speed limits. They may also include location-dependant (advisory) speed limits. It will become increasingly possible to include dynamic speed limits that take account of the actual circumstances at a particular moment in time.

How effective is ISA?

The experiments in both Sweden and the Netherlands show a positive effect of the system. Driving speeds with ISA were slower and more homogenous. In Sweden, no differences were found between the two tested systems.

There has also been a lot of research using driving simulators. For example, in Britain [9][10], the effects of ISA on speed behaviour have been studied in a driving simulator. They tested three ISA systems: an open, advisory system; a combination of half-open/closed system at a voluntary basis (on-off switch); and the same system at a mandatory basis (without on-off switch). They also looked at three different types of speed limits: fixed, variable and dynamic. Based on the speed data, the effects on the number of accidents have been estimated. The estimates show that all systems had a positive effect on safety, with the largest effect of the mandatory system based on dynamic speed limits. Besides the safety improvement, Carsten and Fowkes claim ISA also leads to a reduction in fuel consumption.


Does ISA have negative side effects?

No negative side effects of ISA were found in the experiments in Sweden [5]. However, there is still some concern about this point. Issues that have been reported include:

  • Compensation behaviour: there are indications that drivers compensate by driving faster on road segments where the ISA system is not active [50].
  • Diminished attention: ISA can result in reduction of attention for the road and traffic situation, when the system is not active. This diminished attention expresses itself in, for example, forgetting to slow down when entering a lower speed zone or to accelerate when entering a higher speed zone, but also in forgetting to use the direction indicator [12][65]
  • Over confidence: it is possible that using ISA could result in the driver completely relying on the speed limit indicated by the system, and insufficiently observing the real-time circumstances.
  • Feeling frustrated: the speed limiting by ISA can produce frustration in the driver and in other traffic [64][12].

At present there is insufficient insight into the size of these possible negative side effects and their consequences.

It is generally considered that the current focus should be on developing appropriate principles for human machine interface systems, while various forms of automated control systems, including for example systems such as electronic stability control, are developing, rather than on regulation.


How acceptable is ISA?

According to the SARTRE survey (2004) around a quarter of the European drivers are of the opinion that it is 'very useful' to have a device in the car that restrains you from exceeding the speed limits. This is a little bit lower than for devices preventing drink-driving and driving when fatigued (for both 32% is of the opinion that it is very useful). The practical experiments in both Sweden and the Netherlands have shown that the acceptance of ISA increases if concrete experience with it has been gained.

To assess the political acceptance of ISA systems, the EU-funded PROSPER project performed a survey among different stakeholders (politicians, governmental institutes, research institutes, pressure groups and commercial groups) in eight EU countries. It is reported, that despite differences between countries and between different stakeholder groups, ISA is generally seen as an effective safety measure. An introduction among all driver groups, on all road types and on a mandatory basis is preferred. A half-open system or the active accelerator was considered to be the best option for now. According to the stakeholders, this scenario would result in the best results for safety, environment and congestion. Barriers to the implementation of ISA that were identified included the technical functioning, applicability to the whole network, benefits for the road users and liability issues [4].