Speed perception also depends on cues in the visual environment, thus a proper design of the road environment may bring drivers’ speed choice and expectancy more in line with what is considered to be appropriate in the given circumstances. Then drivers have the idea that speeding is not appropriate, and they do not feel they are forced to decrease their speed but show the appropriate speed behaviour rather voluntarily. Below you find an overview of current speed reducing measures (MASTER Final Report, 1998): • Urban areas:
− Speed humps and traffic calming zones, esp. in residential areas; − Road surface treatments;
⇒ negative effects: decreased driving comfort at low speeds, abrupt braking patterns, increased noise levels;
− Converting traditional junctions into roundabouts. • Rural single-carriageway roads:
− Transverse road markings or transversely placed rumble strips: if the distance between transverse markings or rumble strips decreases while approaching the dangerous location, this usually leads to a reduction in speed, since this creates an illusion of acceleration;
− Longitudinally placed rumble strips that reduce comfort when taken with high speeds require more accurate lane keeping, which is decreasing driving speed;
− Advisory speed signs as a warning for a dangerous location will only lead to speed reductions if drivers understand the reason for the warning.
• Urban and rural roads:
− Decreasing visibility distances: increase uncertainty, need to slow down in order to achieve better anticipation; can be achieved through the amount of curvature, rising and falling gradients, buildings and overgrowth;
− Disadvantage: decrease safety when drivers do not reduce speed; advisable to combine with e.g. road markings or transverse rumble strips to warn drivers to slow down. • Motorways:
− Concept of design speed: speed drivers can maintain comfortably under favourable traffic and weather conditions; drivers have a high level of acceptance about this and behave accordingly;
− Variable Message Signs are more and more used to regulate and guide traffic on motorways with dynamic speed limit signs, e.g. dependant on visibility conditions, a fog warning is given together with an appropriate speed limit.
Most road design adaptations lead to the best speed-reducing effects if they are combined with other adaptations in road design. By providing drivers with the idea of an increased risk for high speeds, driving speeds can be reduced on all kinds of roads. Ideally, drivers’ perceived
(subjective) risk should reflect correctly the actual (objective) risks, or even overestimate them. Reducing road width asks for accurate steering behaviour and increases the perceived risk of running off the road or hitting other vehicles. Placing obstacles along the side of the road works much in the same way. The problem with these measures is that they also increase the actual risk of running off the lane and colliding with an oncoming vehicles or an obstacle for those drivers who do not reduce their speed as intended. Ideally, only the perceived risk of high speed driving should be increased. One way to achieve this could be the reduction of lane width without reducing pavement width.
Currently the largest reductions in driving speed are realised with speed-reducing measures that physically restrict driving at high speeds such as speed humps and chicanes. Roundabouts have primarily a positive safety effect.
Effects of cognitive road classification on driving speeds
Physical measures only force road users to reduce speed, but do not let them choose this voluntarily. Therefore, a better solution is to design roads that are self-explaining. By designing a road that provides a speed image, which is in accordance with the actual speed limit, drivers are persuaded to choose appropriate driving speeds more or less automatically. The Self- Explaining Roads (SER) concept advocates a traffic environment that elicits safe behaviour simply by its design. Therefore, it is important that the function and use of roads match the way people subjectively categorise these roads. At present, subjective road categories do not seem to correspond with the official road categories.
According to the SER concepts, road users classify road scenes into categories. Each type of road has its prototypical representation, so individual roads do not have to be stored (and
remembered) separately. Prototypical representations of road scenes develop through experience and constitute the basis for categorising road environments. As soon as an unknown road is encountered, existing schemes and their typical characteristics are used to categorise this road as a member of a subjective category. In order to obtain Self Explaining Roads, it is important that the design of the infrastructure is adjusted to the way the road environment is categorised in the heads of its users. Successful categorisation may lead to a timely anticipation of possible events. Inadequate categorisation induces wrong expectations that may lead to human errors in
perception and judgement on appropriate speed choice and consequently in an increased accident risk (MASTER Final Report, 1998)
Conclusion
Road category and driving environment apparently influence drivers’ speed choice. It is therefore important that road design gives a correct impression of the road category to drivers, and thus enhances their chances for appropriate choice of speed. In practice, road categories are defined by a set of characteristics on several dimensions such as lane width, road surface, delineation, and elements along the road. Due to local circumstances, some of the predefined characteristics cannot always be applied in a given traffic situation. Providing redundant information does not help the individual road user, since he only makes use of a few of the
only possible if the few dimensions that are used by these road users always provide consistent and correct information on the type of road. It appears to be best to identify those dimensions in such a way that differences between categories are learnt most easily. Only specific dimensions such as lane width and the presence of bicycle lanes seem to affect drivers’ speed choice directly. (MASTER Final Report, 1998)