Generally speaking, ITS and ADAS have made cars safer, but the effect of the various systems differs greatly. Some systems do not help at all or only slightly, while the effect of other systems is unknown, and only a few have a significant (positive) effect on road safety. ITS and ADAS with a significant negative effect on road safety have not been observed yet, although some systems may lead to distraction or reduced alertness (also see the question What are possible disadvantages of ITS and ADAS?). Systems are mainly intended to increase comfort and to control traffic flows, and significant effects on road safety should therefore not be expected. Nevertheless, these expectations do exist, as is the case with Adaptive Cruise Control en navigation systems.
By and large, autonomously intervening systems are more effective than advisory or warning systems. This was shown in studies of the effectiveness of an advisory versus an intervening form of intelligent speed assistance [4], also see the question What is Intelligent Speed Adaptation (ISA) and what effects does it have on road safety?). An example of an autonomously intervening system is Electronic Stability Control (ESC). For each car wheel, the system measures whether the vehicle is going in the direction the driver is steering to. In case of deviation, the car may start skidding. By applying the brakes to individual wheels, this is prevented. Of course, this may not always work; speeds may be too high or road surfaces too slippery. Research has shown that ESC has a profound effect on single-vehicle crashes and a significant effect on multiple-vehicle crashes (amongst other [5] ). A conservative estimate for the Netherlands is that ESC would result in a 30% reduction in single-vehicle crashes and a 17% reduction in fatal multiple-vehicle crashes [6]. Since 2014, ESC has been mandatory for new cars in Europe.
Sometimes, however, a combination of systems is most effective. American research by insurance companies [7] shows that a combination of Forward Collision Warning (FCW; warns drivers when they do not maintain enough headway to the vehicle in front) and Autonomous Emergency Brake (AEB; automatic braking when the driver fails to respond to the FCW signal swiftly enough) results in a 40% reduction of injury crashes in the case of rear-end collisions, whereas solitary FCW did not have an effect on (rear-end) injury crashes.
