Excess speed (above the limit) and inappropriate speed (too fast for the conditions) are major factors in road safety. Increased vehicle speed leads to both greater accident risk and a greater likelihood that the outcome will be more severe – more likely to result in serious injury or fatality.
The so-called “power model” provides a good rule of thumb on how road traffic speed relates to accident risk and severity. The consequence of a small increase in speed is a disproportionately high number of accidents. A good approximation is that injury accidents change in relation to the average speed of road traffic. Injury accidents change with speed squared (v2), serious injury accidents change with speed cubed (v3) and fatal accidents change with speed to the fourth power (v4). This means that reducing traffic speeds by even a small amount will have a large effect in reducing the severity of injuries.
The variants of the power model have been developed from data obtained from countries with comparatively safe roads. Countries with poor quality roads, vehicles with few safety features, a large proportion of two-wheelers and low levels of road user compliance with regulations - may experience a much steeper relationship between speed and accidents.
Variability and large differentials in speed cause disturbances in traffic flow and increase risk. For instance:
Speed management is defined by the OECD as “a set of measures to limit the negative effects of excessive and inappropriate speeds in the transport system.” This requires a strategic approach to the problem of speed - starting with setting appropriate speed limits for different categories and qualities of roads and putting in place a variety of measures that can be used to deliver compliance.
Speed management is generally a central part of a region’s road safety strategy because of the crucial role that speed plays in determining accident risk. Multiple stakeholders are involved, including central and regional government, road operators, the police and road authorities. (See Incident Response Plans)
The variety of measures used in speed management include:
ITS makes possible the use of in-vehicle systems to encourage drivers and riders to comply with speed limits and choose speeds appropriate to road conditions.
Intelligent Transport Systems have a significant role in delivering speed management. For certain types of application, they are crucial. For example, they make it possible to deliver:
Increasingly, both weather-related systems and controlled motorways tend to be fully automated. They involve a range of ITS technologies and systems using distributed sensors to capture and send information to central traffic control centres for display on roadside information panels – as well as speed cameras for enforcement. (See Weather Management)
With the growth of real-time communications into nomadic devices and vehicles, we are likely to see greater delivery of information and warnings conveyed within the vehicle directly to the driver. It is already the case that many commercial satellite navigation systems and navigation applications for smartphones provide information on speed limits and can be set to warn the driver about speeding. It is in the interests of road authorities to provide suppliers of digital road maps with up-to-date information on speed limits and in particular with timely information on changes to speed limits.
ITS technologies can assist in:
United Nations Road Safety Collaboration has produced Speed Management: A Road Safety Manual for Decision-makers and Practitioners. This is available on-line at http://www.who.int/roadsafety/projects/manuals/speed_manual/en/. Chapter 3 covers tools, including ITS tools such as Intelligent Speed Adaptation(See Intelligent Speed Adaptation).
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 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 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 (http://tn-its.eu/).
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.
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 - http://archive.etsc.eu/documents/Intelligent_Speed_Assistance_FAQs_2013.pdf