Road design as a primary aid to speed enforcement?

Letty Aarts, senior researcher, SWOV institute for road safety research, the Netherlands, discusses how road design can act as a primary aid to speed enforcement

There are a number of infrastructural measures that can be implemented to improve drivers' behaviour and support enforcement of road traffic law. The objective of these is to prevent most violations from occurring, influencing unintentional behaviour or at least preventing errors and unintentional violations, leaving the police and enforcement agencies to better target resources towards drivers who still and/or severely violate the rules. Infrastructure can support drivers by providing cues that are meant to affect both conscious and unconscious behaviour. By providing continuous feedback about location-specific rules, drivers are provided with instantaneous and explicit knowledge of what rules are in place, for instance when they have missed the posted speed limit.

A logical fit of road layout and location-specific rules provides support for better and more natural or intuitive compliance. This fit can also support the predictability of rules and the behaviour of other road users on a road ('Recognisable Roads'), and so the implicit expectations of drivers. Research has shown that supporting expectations of drivers decreases the probability that they miss important information and make errors that can result in a crash. This idea of implicit and explicit information by road design and layout is linked to the concept of Self-Explaining Roads (SER). In the Netherlands, the 'predictability of roads' concept is preferred. Figure 1 shows the chain of elements involved.

Supporting driver expectations

The Netherlands is one of several countries that have looked to make roads more informative for drivers. This started within a framework inspired by the national Sustainable Safety vision. The core of this vision is to prevent severe crashes from happening and to decrease the probability of severe injury in the case of a crash by an integral human-centred approach of engineering, education and enforcement measures.

The elaboration in the Netherlands of informative, predictable and recognisable roads is unique as it is based on a limited number of road categories. The general principle is that, ideally, roads have only one (a flow or an exchange) function. Road layout facilitates homogeneous use in speed, mass and direction that fits safely to the function. This also holds for the forgivingness of roadsides and results in three different road categories.

High-speed (100 or 120km/h) through roads feature physical separation of the driving directions. Slow, vulnerable road users are prohibited and there are access restrictions for mopeds, cyclists and agricultural vehicles. Intermediate-speed (50 or 70km/h urban and 80km/h rural) distributor roads feature intermediate speeds on road sections and low speeds at intersections. Physical separation of slow and vulnerable road users and fast traffic is preferred and there are some access restrictions.

On access roads, the mixture of all traffic types requires low speeds (30km/h urban, 60km/h rural), which are also enforced by the road layout. The unique design characteristics of each have to be recognisable.

Implementation started with road authorities categorising their networks. Subsequently, Essential Recognisability Characteristics (ERCs) were defined in a design guideline which was the start for large-scale adaptation of the traditional (less informative) road layout into something more informative and recognisable. Figure 2 shows rural road design.

Expectational influences

The ERCs comprise combinations of roadside markings and separation of driving directions. These are evidence-based road layout elements and can be quite easily implemented in practice. Before the ERCs were defined the road design elements that would help make roads distinguishable and eventually self-explaining were studied.

Studies were mainly performed with manipulated photographs of roads and found that, in general, distinction between road categories is enhanced when the differences among categories are sufficiently large and when the variation within each category is not too large. Roadside chevrons were found to make roads better distinguishable from traditionally designed roads. It has also been suggested that speed colour coding may enhance the distinctiveness of road types, and red cycle lanes were found to be self-explaining. Motorways turned out to be very recognisable as high-speed roads with access restrictions for slow traffic, when the road is equipped with emergency lanes, safety barriers and gantries. Low-speed zones, in their turn, are easily recognised as such when they are characterised by a credible low-speed design (a narrow and curved road, with uneven surface, and a built-up area near to the road).

Characteristics that could add to the recognisability of roads had to be continuously visible, practically implementable and feasible (which ruled out the use of lighting as this is added to only a very few roads), not detrimental to road safety and visible in all conditions (so not causing a road surface to become slippery, and being visible during darkness and adverse weather). This led to edge markings and separation of driving directions being adopted.

After ERC implementation, it turned out that access roads could be distinguished quite well. Through roads and distributor roads, however, were often mixed up by drivers. Probable reasons for this are the variations in road layout that are still allowed within each road category (Figure 3) and the use of side markings as distinguishable characteristics between these road categories, although it is not this characteristic but the separation of driving direction which is more meaningful to drivers. Explicit information about the meaning of the road layout improved performance (this is particularly so for the green centreline used on regional through roads; it is a salient, unique road layout element which is however not self-explaining for road type and its comprising characteristics). Sufficient distinction between road categories, using salient road layout elements, also turns out to be important for making transitions between road categories and the change in expectations and behaviour noticeable for road users.

In short, implementation of the ERCs in the Netherlands has been a step towards improving the recognisability of road types. As the ERC guidelines still allow for a lot of layout variation within road categories, consistency is an issue that has to be improved.

Behavioural influences

A huge body of work has shown the behavioural effects of road layout elements. Most studies are related to speeding behaviour and the concept of 'credible speed limits'. As a starting point, speeds should be safe and supported by a credible limit. The latter is defined as one which matches the image that is evoked by the road and the traffic situation. Credible speed limits have been found to be influenced by: primary credibility factors, which physically force drivers more or less to adapt their speed (straightness of a road and physical measures such as speed humps); and secondary ones (the density of elements near to the road, road width, and type and quality of the road surface). Secondary factors are related to the speed feedback they give. The more signals the environment provides, for example because of a large number of trees along the road or uneven surface, the higher the perceived task demand and the higher the probability that driver will adapt their speeds.

A study in the Netherlands of credibility of speed limits on provincial single-lane rural roads with a speed limit of 80km/h roads showed that free chosen self-reported driving speeds differed largely (speeds of 75 to 95km/h were reported). Layout characteristics related to speed choice included: sight length; road and lane width; density of the vegetation and buildings along the road, particularly on the driving side of the road; and separation of driving direction, being easily over-rideable (markings) or not (barriers).

These findings have given rise to discussion among road authorities about how uniform layout within a road category has to be. Related to this is which essential characteristics can be defined that cost a minimum of effort without having adverse effects on road safety.

As the implemented ERCs are also meant to lead to more homogeneous behaviour, the behavioural effects were studied too, mainly using driving simulators to control other influential road characteristics. A study that tested the effects of traditional road layout, ERC layout and a more physical layout that meets the standards of a safe system design (Figure 4), found that the more recognisable and safe layout led to lower speeds and to a lateral position towards the edge of the road. It is however important to notice that these findings are the combined result of recognisability and the behavioural effect road elements evoke directly by their presence. In the studied layouts, lane width, for instance, was smaller by markings (ERC) or barriers (Safe System design) than in the traditional layout, and this is known to reduce speed even if not consistently adopted per road category.

Recognisability and concepts like credible speed limits are meant to act on both the explicit knowledge of rules and intuitive behaviour. In this way, it should evoke compliant behaviour in the majority of road users, and give the police a better starting point for the enforcement of rules against those drivers who still don't want to comply.

The principle of recognisability of roads and predictability of road layout and behaviour has been elaborated in the Netherlands, based on scientific evidence and practical considerations. Implementation started in 2004 and will continue. Although the consistency of road elements within road categories as well as behavioural and direct safety effects of the chosen characteristics are issues that show room for improvement, implementation of the ERCs has improved the situation for drivers: they now have a better opportunity to know which road type they are driving on and what speed to adopt.