The first step in the process of moderating speeding behaviour is to develop legislation which specifies the speeds which are applicable to different types of road sections. The implementation of such legislation is an essential element of all enforcement based speed control strategies as it provides motorists with a clear indication as to what is deemed to be law abiding speed behaviour. Enforcement of this legislation can also increase the efficiency of policing operations as it allows the speed of the motorist to be used as the sole means of determining illegal driving behaviour.
4.2.1 Speed limits
The primary legislative means of imposing some control over the speeds at which motorists travel is through the use of speed limits. The primary function of speed limits is to provide drivers with information regarding what is considered to be the safest speed for the road environment in which they are travelling. Speed limits can be specified as 'absolute' (the maximum speed at which a driver is permitted to travel) or 'prima facie' (the speed above which a driver would have to prove that their speed was compatible with prevailing conditions). Absolute speed limits are more commonly adopted than 'prima facie' speed limits. They also appear to exert a greater influence on the distribution of speeds and have been reported as resulting in less excessive speeding behaviour (Fildes &
The introduction of maximum speed limits has significantly increased the effectiveness of speed enforcement operations by providing police with a simple means (travel speed) of determining inappropriate speeding behaviour (Evans, 1987). This was considered to be a marked improvement on traditional enforcement techniques which relied upon the subjective judgement of a police officer to determine if speeding behaviour was inappropriate or dangerous for the existing road environment conditions. In many situations this was difficult to prove and often relied on the occurrence of some type of road accident.
Although speed limits are often associated with police enforcement activities, they can be an effective speed countermeasure in their own right. Plowden and Hillman (1984) stated that speed limits provide legislators with a powerful tool not only for modifying speeding behaviour but also for reducing the number of road traffic accidents. In a comprehensive review of speed limit control, they concluded, that the international evidence has shown that the number and severity of road accidents increases in relation to vehicle speed and that substantial reductions in road accidents are brought about by imposing or lowering existing speed limits.
The literature relating to the effect of speed limit changes on driving behaviour and accident and fatality rates has been reviewed in the previous section relating to speed and accidents. The general conclusion from these studies was that lower speed limits result in a reduction in mean travel speed and a corresponding reduction in the number of road traffic fatalities. Although speed limits are commonly regarded as an effective speed management tool they can actually result in road safety problems especially when motorists do not believe that the posted speed limits are appropriate for the road environment in which they are travelling.
Reasons for setting speed limits
Although, in the majority of Western countries, speed limits have been applied in some form or other for many years, it was the international oil crisis in 1973 which provided the main impetus for the widespread use and implementation of speed limits. This crisis led many countries to develop a range of energy conservation measures which included legislation for the introduction of new or lower speed limits. A somewhat unexpected result of the speed limit changes was a significant reduction in the number of road accidents, and associated injuries and fatalities. As a result of the reduction in the level of road trauma the use of speed limits became recognised as an effective road safety countermeasure.
However, speed limits are not set solely on the basis of safety. The mobility needs of society must also be taken into account. The underlying principle of speed limits is to optimise both these aspects to the best degree possible, bearing in mind that ultimately, mobility and safety requirements will often be in conflict (Fildes & Lee, 1993). The most appropriate speed limit is one where the majority of drivers believe that authorities have made an acceptable compromise between their mobility needs and the need to maintain a relatively high level of road safety.
Fildes and Lee (1993) stated that balancing these two needs can be extremely difficult. They suggested that the mobility needs of the driver are continually changing. In rational terms, mobility is a function of the vehicle and roadway, trip purpose, duration, economic consequences, utility, and so on. Anyone of these variables may change from moment to moment thereby altering the immediate mobility needs of the driver. However, a driver's
perception of safety (accident or injury risk) is not always immediately apparent from an increase in travel speed because accidents are relatively rare events. They concluded that it is therefore not surprising that authorities are often accused of erring on the side of safety. That is, they are often criticised for setting speed limits that do not always reflect a credible balance between the many factors affecting speed choice under optimal driving conditions.
Setting speed limits
Speed limits are traditionally set using the 85th percentile method which is defined as the speed at or below which 85% of motorists choose to travel. The method was introduced as a result of research undertaken by Witheford (as cited by Fildes & Lee, 1993) who stated that the 85th percentile speed is that most desirably approximated by a speed limit. He argued that because of the general straight and steep slope of the typical speed distribution below the 85th percentile, a speed limit set only a little lower will result in a large number of drivers violating that limit.
The benefit of using this method is that it reflects the speeding behaviour of all motorists due to the fact that it is based solely upon the speed limit that motorists perceive to be acceptable. It also allows enforcement activities to be targeted only at those motorists at the high end of the speed distribution and, as such, it is generally accepted by the public and does not place an excessive burden on policing resources. However, Fildes and Lee (1993) indicate that the use of this method may not provide a true reflection, or an unconstrained measure, of the speed that the majority of motorists perceive as being acceptable. They suggest that a driver's choice of speed is never truly unconstrained and that the 85th percentile is actually an expression of the speed at which motorists are prepared to travel on that road, influenced by factors such as the prevailing speed limit, enforcement activity, the amount of traffic and time of day.
A number of researchers have recommended the use of alternative methods of setting speed limits. Nilsson (as cited in Cameron, 1993) has suggested that speed limits be set on the basis of transport economics, where accident and injury cost savings must compete with travel time and vehicle costs. He suggested that this would allow speed limits to be set in relation to some predetermined level of acceptable road trauma. This proposition has been supported by a number of other researchers (Cowley, 1981; Salusjarvi, 1981) who have suggested that setting speed limits based upon reductions in transportation costs would not only reduce the level of road trauma but would also result in significant financial savings.
Problems with speed limits
One of the main problems associated with speed limits is that motorists often perceive them to be inappropriate for the existing road environment conditions. The credibility of speed limits plays an important role in the process of encouraging safe driving speeds and it is generally accepted that speed limits should match the expectations of drivers to some degree (Schnerring, 1985). Credible speed limits ensure that there is an adequate level of voluntary speed compliance which in turn reduces the reliance on limited police resources to enforce speed limits. However, if motorists believe that speed limits are not appropriate then the likely result will be an increase in the level of speeding behaviour. Ruschman et al. (as cited in Fildes & Lee, 1993) claimed that excessive speeding can, in part, be attributed to improper speed limits. Drivers may believe that their behaviour in exceeding the speed limit is not risky and they therefore do not perceive it as being illegal.
The aim of authorities is to set speed limits that are not only appropriate to the road environment but are also consistently applied. Doing this educates drivers to understand the reasons why certain speed limits apply in different situations and allows them to identify speed zones based upon previous experience of the relationship between speed limits and road environment (Jarvis & Hoban, 1988). However, the criteria for setting speed limits can be extremely complex and the result is that there are often many inconsistencies in the speed limits which are applied in different road environments.
One such inconsistency relates to the discrepancy between posted speed limits and 'design speeds' for particular types of roads (Fildes & Lee, 1993). Roadways are often designed by engineers to accommodate higher speeds than those stipulated by the posted speed limits. The primary reason for this engineering design practice is to provide the driver with a certain 'safety' margin. However, because these roads are designed to accommodate higher speeds it is not surprising that many drivers exceed the posted speed limit and travel at a speed closer to that for which the road was designed. Fildes et al. (1991) have also reported that some speeding motorists (who were not intentionally speeding) found it quite difficult to maintain their speed at the legal limit. Their research questioned the validity of building roads with high design speeds, stating that effective speed management on these roads places an unnecessary strain on enforcement resources.
There are a number of ways in which authorities may be able to improve the credibility of speed limits. A number of possible solutions, including the use of 'expert' speed zoning systems and the introduction of some form of variable speed limit system, are detailed in a later section within this review. Regardless of the perceived credibility of speed limits there will inevitably be a number of road users who choose to travel at speeds greater than that nominated. In such situations enforcement may be the only possible solution for deterring these motorists.
Traffic law enforcement is generally regarded as the final step in the process of moderating the level of speeding behaviour. If the road user cannot be encouraged to drive at a speed which is appropriate for the road conditions, or which corresponds to the specified speed limits, then reliance is typically placed on the law enforcement system to deter, apprehend and punish. To provide a review of speed control strategies, as they currently operate is, by and large, equivalent to examining the impact of policing operations and legal sanctions on the speeding behaviour of road users. There are two commonly adopted approaches to speed enforcement. The first approach involves the use of traditional enforcement methods and the second approach involves the use of automated speed detection devices. These two approaches differ in a number of fundamental ways and each will therefore be reviewed in two separate sections.