Speed Management

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:

• large speed differences between vehicles result in them quickly closing in on each other with reduced time to impact (smaller safety margins)
• large differences in vehicle speeds on successive road sections are problematic, particularly where traffic has to slow down abruptly following a relatively straight and high-speed section of road. This highlights the importance of consistency in road design and speed management – to encourage better and more predictable driver behaviour

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:

• road design, including both layout and markings – such as the principle of the self-explaining road where road users can easily understand the function of the road, how they should drive and what to expect of other road users
• regulation, including rules on which categories of vehicle are permitted to use a road and what the speed limits are for particular vehicles
• driver training
• safety campaigns
• speed enforcement

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.

The role of its in speed management

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:

• real-time roadside weather-related warning systems
• or controlled (“smart”) motorways on which speed limits change in response to traffic flows (See Case Study: Active Traffic Management)

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:

• encouraging compliance using roadside graphic variable message signs to provide feedback to individual drivers on their speed
• compensating for deficiencies in road design by advising drivers to slow down at certain locations such as the approach to sharp curves
• informing drivers and riders about real-time conditions and dynamic speed limits
• encouraging compliance through camera enforcement (See Policing/Enforcement)

Further information

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).