Safety evaluation of measures for the separation of oncoming traffic in median strips on federal highways

Abstract

Currently there are 17.000 accidents due to contact with the central reserve or safety barriers at the road perimeter on federal motorways each year (time of observation: 2004 to 2006) in Germany, resulting in personal injuries and heavy material damage. An essential aim of this assignment was the gathering of information about the volume, characteristics and results of accidents generated by a breakthrough of the central divide. A breakthrough is defined as an accident, where one or several vehicles or part(s) of one or several vehicles or parts of the central divide end up on the opposite side of the motorway. Also, accidents where a collision with equipment on the central divide (e.g. signposts) has occurred, are covered in this study. During the period of 2004 to 2006, which is forming the database of this assignment, 263 breakthroughs occurred on German motorways, meaning an average of 88 per year. In the course of 70 annual accidents, a heavy vehicle has broken through the central barrier, in about 20 percent of this cases a breakthrough accident occurred involved one or several cars. Thus, about 0.5 percent of the annual rate of about 17.000 accidents with a contact to beam barrier on the central divide or the (in driving direction) right hand side perimeter of the road are defined as breakthrough accidents, meaning that the breakthrough of the central divide is a relatively rare occurrence. The analysis of breakthrough accidents demonstrated that there is a fairly even distribution of this accident type throughout Germany, while there is also no correlation between heavy truck traffic volume and the frequency of breakthrough accidents. About 35 percent of breakthrough accidents only involve one vehicle. About 75 percent of the breakthroughs involve the deviation of the vehicle from the carriageway – 49.2 percent with deviation to the left and 25.8 percent with a sequence involving a deviation to the right with a consecutive breakthrough through the central divide. Most of the accidents do not involve a collision with another vehicle or due to skidding (about 92 percent). In case of previous collision, this event occurred before contact with the beam barrier. The breakthrough accidents covered in this study primarily result in light casualties (about 37 percent), followed by accidents with heavy material damage (about 27 percent), accidents with heavy casualties (about 26 percent) and accidents with fatalities (26 accidents, equating about 11 percent of all breakthroughs). In case of a fatal breakthrough accident, usually more than one fatality was involved, resulting in an eight- to nine times higher fatality rate of breakthrough accidents compared to average accidents on motorways. Another aim of the assignment was the deduction of implementation guidelines for beam barriers with a very high containment capacity on the central divide of motorways. In this instance it was found out that in just 42 (out of a total of 240) cases the infrastructure was causally connected with the breakthrough event, equating in a 17.5 percent share of all breakthrough accidents. These cases typically cover configurations, where adverse weather conditions (wet or frozen road surfaces) in combination with not conform road alignment parameters have resulted in an accident. Compared to the complete accident volume on German motorways, a more than average number of accidents have occurred on road segments with a distinct decline or incline, especially in case the decline was also situated in a curve as well as in a road sector with three lanes. A significant number of accidents were counted at motorway intersections with shorter than regular acceleration or deceleration lanes as well as at intersections in combination with another “special” characteristic (e.g. a curve or a decline). It has to be observed, however, that the frequency of such road segments without any breakthrough events is unknown. Therefore and under consideration that the spatial distribution of breakthrough accidents is fairly even among German motorways, a global deduction of implementation guidelines remained impossible as part of this study. Nevertheless the use of barriers with a very high retention potential can be taken into consideration for road sections, where deviation accidents of truck traffic have been observed and those featuring special characteristics as defined by this study. In these cases it has to be observed that even barriers with a very high retention potential are not generally capable of preventing all breakthroughs, as demonstrated by four breakthrough accidents involving concrete barriers between 2004 and 2006. Also, the accident analysis has illustrated that the collision with concrete barriers usually results in an increased injury probability for all vehicle occupants. Thus a recommendation of a system-wide general use of barriers with a very high containment capability (most notably inflexible steel beams or concrete casings) cannot be regarded as purposeful.