European Sight Distances in perspective - EUSight: Literature review report

Deliverable 2.1 of CEDR Call 2013: Safety


Petegem, J.H. van; Schermers, G.; Hogema, J.; Stuiver, A.; Broeren, P.; Sterling, T.; Ruijs, P.; Weber, R.




Part of the CEDR Transnational Road Research Programme Call 2013: Safety is the research project European Sight Distances in perspective – EUSight. The objective of this research project is to conduct a detailed examination of the subject of stopping sight distance (SSD) and its role and impact on highway geometric design, taking into account differences (and similarities) between EU Member States.

Sight distance (SD) means the unobstructed visibility that is needed to be able to safely and comfortably perform the driving task and to avoid conflicts or collisions with objects or other road users. Stopping sight distance (SDD) means the distance over which a driver needs to be able to overlook the road to recognize a hazard on the road and stop his vehicle in time.

This report describes the result of Work package 2 of the EUSight project. It describes both an international literature study and a review of road design guidelines for motorways of a selection of EU member states on SD and SSD related aspects. This research considers stopping sight distance from different (related) aspects: human factors (‘the driver’), road characteristics, vehicle characteristics and conditions (like wet, darkness or environment).

The literature review revealed that there are many studies available on Perception–Reaction Time (PRT). Obviously, there are differences in PRT between and within drivers. Hence, PRT is characterised by a distribution rather than by a constant value. For SSD, the common approach is to use percentiles of the PRT distribution. The 85th percentiles that have been reported range from 1.4 to 1.9 s; 90th or even 99th percentiles may range from 1.8 to 2.5 s. But which percentile should be used for SSD calculations does not follow from the literature review. Ultimately, this is a trade-off between safety and comfort on the one hand and cost/space travel time and adaptation to landscape on the other.

Brake assist and similar systems can help improve the response time of the vehicle and of the brake performance of the driver-vehicle system. It can be expected that more of these systems will become available over the coming years. Still, for the years to come, SSD criteria have to be based on a vehicle fleet containing vehicles without such systems. As a consequence friction is speed dependent.

The classical road condition used in SSD calculations is a wet surface. As a consequence, the road friction should be considered as a function of speed and of water depth. Further, at higher speeds, the friction coefficient is a function of the tyre tread depth. The existing surface types (concrete / dense asphalt / porous asphalt) are characterised by different micro and macro structures and by different water draining characteristics, accumulating to different friction coefficients in rain. These characteristics should be taken into account when choosing SSD parameters later on in the project.

The review of the road design guidelines for motorways on SSD considered the guidelines from Denmark, France, Germany, Ireland, the Netherlands, Switzerland and the United Kingdom. It was found that the SSD requirements are very similar for most countries except for the UK and Ireland, where the preferred SSD requirements are about one third higher than for the other countries. The UK and Irish guidelines give little or no insight into driver reaction times, deceleration values, braking coefficients, etc. and therefore these differences cannot be explained from just a guideline review. However, indications from the literature review indicate that the UK has adopted a PRT value that is comparable to those used in other countries but the standard deceleration rate applied may be conservative by comparisson. For the remaining countries there are some differences concerning the details on driver, vehicle and road characteristics, although these differences are most often small and thus reslultsin similar values for SSD.

Considering the small differences between the SSD requirements it can be concluded that there is a wide spread consensus on the SSD requirements between the country design guidelines. As the oldest guidelines of the selection reviewed is dated in 1983, it can be concluded that the requirements have not changed much over time.

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CEDR, Brussels