Abstract
Intelligent Speed Adaptation System (ISA) is the name of those systems that supervise the vehicle speed and compares it with the legal or safe speed. Depending on the permissiveness with the driver actions, these systems can be classified in 3 groups: advisory, voluntary and mandatory. Another classification is based on the variability of the speed limits, which can be fixed, variable and dynamic.
In the United Kingdom, a statistical methodology was developed to estimate the effect of these systems on road safety and accident reduction. The estimations are based on statistical analyses. This methodology is now applied to the Spanish case. The study is based on considering that the speed distribution suffers two important effects taking into account the type of ISA system introduced: a translation of the speed distribution and a transformation of the speed distribution. Besides, it is studied the effect of using an ISA system only in nonbuilt- up areas, only in built-up areas or in both areas.
The results, applying this methodology to the Spanish case, show that 36.6 % of injury accidents and 59.8 % of fatal accidents can be avoided if a mandatory ISA system with dynamic limits is implemented in road vehicles and in all types of roads. This is the best solution but it is also the most complex and expensive one. Simpler solutions provide lower accident reductions: the advisory ISA system with fixed limits provides significant accident reductions (11.4 % of injury accidents and 21.5 % of fatal accidents). A compromise should be achieved between cost, accident reduction and social acceptance because, although most of the drivers that have used an ISA system recognize that safety is improved, few of them accept a mandatory system that controls the speed of their vehicle when the safe or legal limits are exceeded. An advisory ISA system with dynamic limits can solve this acceptance problem and reductions of 16.9 % of injury accident and 31.0 % of fatal accident would be achieved if the system is applied to all types of road in Spain. These percentages are reduced to 13.3% and 24.9 %, respectively, if the system is applied only to non-built-up areas.
Finally, the results are compared with the conclusions of studies in other European countries, such as United Kingdom (studies of the TRL Transport Research Laboratory- and the University of Leeds) and Sweden.
Keywords:ISA systems, statistical analysis, accident savings, ITS, safety