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Mobility & Transport - Road Safety

Road engineering


Road engineering

Overall road design should indicate the function of a road and, in combination with design speed, the appropriate speed limit. At specific locations, additional road engineering measures may be necessary to ensure the safe speed of cars. If applied in a consistent way, this type of measures may also help drivers to recognize the traffic situation and the speed limit. Locations where physical speed reduction measures are often necessary are residential areas, at-grade intersections at main urban and rural roads, high speed to low speed transition zones, and midblock pedestrian crossings.

Use of 30 km/h zones and accompanying measures in the Netherlands

In the Netherlands between 1985 and 1997, about 10-15% of the urban residential roads were converted to 30km/h-zones. Newly developed residential areas were designed as 30km/h zones. The roads in these zones were redesigned using road humps, road narrowings and mini-roundabouts to ensure the speed limits were respected. As part of the Sustainable Safety initiative, there was subsequently a major increase in investment in these zones and between 1997 and 2002 the proportion of roads treated has increased to 50%. Engineering work within the zones was however less extensive than in the earlier zones.

In parallel since 1985, engineering work has also been used on the urban roads where speed limits were not reduced to 30km/h to maximise separation of vulnerable road users and maintain low traffic speeds.

Source: Koornstra et al, 2002

Residential areas: 30 km/h + supportive engineering measures

An increasing number of countries apply 30 km/h zones in residential areas, based on the known relationship between speed and the chance for vulnerable road users to survive a collision. In order to ensure that the 30 km/h limit is not exceeded, the limit is best supported by engineering measures such as speed humps, road narrowings, chicanes and raised areas at intersections. The application of these vertical and horizontal measures has been found to have a substantial effect on speed [66][16][57]. Many studies also found a substantial effect on the number of injury accidents in 30 km/h zones with this type of supportive engineering measures. Elvik [18] performed a meta-analysis on a large number of evaluation studies and reported a 25% reduction in injury accidents. Although this study dealt primarily with traffic calming, similar conclusions can be drawn for 30km/h zones.

Roundabouts and midblock pedestrian crossings

Speed reduction is also particularly appropriate at at-grade intersections at main urban and rural roads. For these locations, the application of roundabouts is a very effective speed reduction measure. In addition, at roundabouts the angle of impact is smaller, resulting in less severe consequences in case of a collision. Based on a meta-analysis [20] report an injury accident reduction between 10-40%, depending on the number of legs and the previous form of traffic control. The largest reduction was found for four-legged junctions with traffic signals before. Fatal and serious accidents are reduced more than slight injury accidents. The effect on pedestrian accidents is similar to that of other accident types; the effects for cyclists are somewhat smaller (10-20%). The meta-analysis showed an increase in the number of damage-only accidents at roundabouts.

At mid-block pedestrian crossings the speed of motorized vehicles should also be kept low. A raised crossing will make high speeds less likely.


Transition zones

When entering the lower speed zone, in particular after a period of driving at a high speed, drivers will easily underestimate their travel speed, and hence insufficiently adapt their speed. Here specific measures help to indicate the transition from one traffic environment to another, to another traffic behaviour, and primarily to another speed [31].

Of special concern is the entrance of a village from a major through road. ETSC describes two principles for measures in such transition zones. The first principle is that complementary measures along the through route within the urban area are required. The second principle is that measures at the transition zone should be such that they achieve a cumulative effect, culminating at the actual gateway to the towns or villages. The latter can be achieved, as the ETSC reports says, by a combination of road narrowing and the introduction of vertical elements, culminating in the gateway. This is an example of a psychological measure that relies on the driver's perception of the appropriate speed: speeds are lower where the height of the vertical elements is greater than the width of the road [31].

Taylor and Wheeler evaluated the effects of 56 traffic-calming schemes in British villages on main interurban roads where the speed on the approach to the villages was typically 90 km/h. It was found that schemes with only gateway measures resulted in a reduction in fatal and serious accidents within the villages of 43%. The number of slight accidents increased by 5%. The accident reduction was higher for pedestrians and cyclists than for motor vehicles. Higher accident reduction rates were reported for schemes with additional measures inside the villages (chicanes, road narrowing, mini-roundabouts, speed humps and speed cushions). Here, the number of fatal and serious accidents decreased by about 70% and the number of slight injuries by about 37%.

The transition between motorways and the adjacent lower speed zones is another situation where underestimation of speed may result in insufficient speed adaptation. A roundabout at the exit of the motorway may restore speed perception and facilitate choosing the appropriate speed.

Where a long straight stretch of road enters a winding section, physical speed reduction measures are less suitable. Currently, roadside warning signs and advisory speed limits are the most commonly used in this type of situations. Vehicle actuated signs warning of speeds being in appropriate for approaching hazards have proved effective in Great Britain [71]. There is also experience with (combinations of) transverse and longitudinal pavement markings at dangerous curves as a perceptual rather than a physical speed reduction measure [23]. Similar pavement markings have been used at village gateways. Evaluation studies generally show a positive effect on driving speed, but there is uncertainty over how long this effect will last over time [42]