Road Network Operations
& Intelligent Transport Systems
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Intelligent Speed Adaptation (ISA)

Intelligent Speed Adaptation (ISA), also known as Intelligent Speed Assistance, brings speed management into the vehicle. The aim of ISA is to discourage or prevent speeding by informing drivers about the speed limit for a road and warning them about excess speed. The most sophisticated systems prevent speeding by way of an electronic speed limiter. The fundamental distinction is whether they are advisory or intervening:

  • advisory ISA systems typically beep at the driver when speeding over a certain threshold is detected
  • intervening ISA systems typically limit speed at or just over the speed limit (although the driver can disengage the speed limiter at any time)

Advisory ISA is already on the market — it is a feature in many commercial satellite navigation systems, although it is generally for the end-user to decide whether to implement the function. Manually set speed limiters are available in many vehicle models. No vehicle manufacturer currently offers a full intervening ISA.

Fully- intervening ISA has been trialled extensively in real-world driving (so-called Field Operational Tests). These trials have produced generally positive results in terms of behaviour, showing that the use of ISA in all its forms brings about a significant reduction in speeding. They also indicate a reasonable level of acceptance by users, even though users might feel somewhat disadvantaged by having ISA in that they can see other drivers travelling faster than they are.

Using well-validated models of the relationship between driving speeds and risk - calculations of the impact of ISA on accidents have been made. Probably the most comprehensive set of calculations is from trials conducted in the ISA-UK project during 2004-2006. The prediction is that an advisory ISA in general use, would save 3% of injury accidents and an intervening ISA would save 12% of injury accidents and 20% of fatal accidents.

In its strongest variant (an intervening version which cannot be overridden), the prediction is that ISA would deliver a 29% reduction in injury accidents. Applying the power model this translates into a 50% reduction in fatal accidents. [Shifting driver behaviour to virtually full compliance with speed limits can cut the number of fatal accidents in half – in a country with good driver compliance patterns. For countries with poorer levels of compliance, the impact would most likely be greater – if drivers accepted the technology. (See Speed Management)

ISA Technology

ISA consists of two major elements or sub-systems – informing the driver (all systems) and controlling the vehicle (only applicable to intervening ISA). A visual display and speaker system also need to be provided. Where ISA is installed as original manufacturer’s equipment, the display and speakers are integrated into the dashboard.


The information part of ISA typically uses a digital road map, enhanced with speed limit information. This can be supplemented with a digital camera on the vehicle that reads speed signs to make up for any gaps in the map. It can also offer real-time information for locations such as work zones.

Digital map providers routinely collect speed limit information and can provide extensive coverage for many countries. Agreements are needed for data exchange between public authorities and commercial map providers to ensure that changes to speed limits are quickly incorporated into maps. One such initiative is the European Transport network ITS Spatial Data Deployment Platform (TN-ITS) which covers a range of road data including speed limits (


Many new vehicles, both cars and trucks, currently feature driver-set speed limiters (cruise control) either as standard or as an option. Replacing the driver control with ISA works without driver intervention and is a straightforward technical step.

ISA is a mature technology and the purchase of cars with ISA or with ISA-like features is being promoted. Many fleet management systems incorporate ISA-like capability – with speed infractions by drivers being reported back to the fleet manager. This is known as “recording ISA”. Similar features are also included in many PAYD (Pay as you drive) or UBI (Usage Based Insurance) schemes.

Further information and reference sources

The European Transport Safety Council (ETSC) has a Frequently Asked Questions (FAQ) section on Intelligent Speed Adaptation which provides a good summary of the current position on ISA implementation -

Reference sources

Nilsson, G., 2004. Traffic Safety dimensions and the Power Model to Describe the Effect of Speed and Safety. Bulletin 221. Department of Technology and Society, Lund University, Lund, Sweden

Elvik, R., Vaa, T., 2004. The Handbook of Road Safety Measures. Elsevier, Boston, USA