Underrun and side protection for trucks

01 Jun 2017 Vehicle Technology
When it comes to vehicle safety, measures to make trucks safer should not be neglected either. Although heavy-duty trucks are only rarely involved in road accidents, their large dimensions and typically open frame design on the sides and at the rear make them less compatible with other road users. For unprotected road users and car occupants, therefore, the consequences of collisions with trucks can be particularly devastating. To a certain extent, measures to improve exterior passive safety such as front and rear underrun guards and side protection have helped to mitigate risks. Although state-of-the-art driver assistance systems for preventing accidents and mitigating the severity of accidents offer by far the greatest potential, these mechanical passive safety systems will remain vital as a “mechanical fallback.”
The lack of compatibility means that an accident in which a car collides with the rear of a heavy-duty truck or trailer can prove fatal. As experts from the German Federal Highway Research Institute found out, six out of ten car occupants involved in this form of accident suffered serious or fatal injuries, with around 30 to 35 car occupants dying in such accidents annually. In relation to the figures for 2015, this corresponds to around 2% of all 1,620 fatally injured car occupants. In the USA in 2015, this figure was as high as 16.1%.
Accidents in which a car collides with the rear of a semi-trailer typically occur on highways. The average speed of the truck is 80km/h and that of the car 125km/h, which corresponds to a relative collision speed of 45km/h in the car.
Fundamental findings from accident investigations and early crash tests at the Technical University of Berlin led to the introduction of rear underride guards back in the 1970s with the support of the German Federal Highway Research Institute. Directive No. 70/221/EEC laid down for the first time in the nations of the then European Economic Community an internationally recognized technical description for rear underride guards. When implemented at national level in the member states, it was generally used as a design specification – for example, through conversion into German registration law in 1975 with the introduction of Section 32b of the road traffic licensing regulations (StVZO): “The underride guard must have the same flexural strength as a steel beam, whose cross-section has a resistance torque against bending of 20cm².
With UNECE-R 58, which was published in 1983 and also recognized outside Europe, regulations stipulating the end result to be achieved were agreed upon. The test procedures, which are still in use to this day, involve applying successive quasi- static forces at five symmetrical load application points (P1, P2, P3; see Figure). In response to ongoing criticism that rear underride guards did not in fact provide sufficient protection in real-life accident situations, the test loads were significantly increased. The requirements laid down in UNECE- R 58-03 currently apply, which means that the test loads for rear underride guards are now greater than those for front underride guards, which became a legal requirement in 2000 with Directive No. 2000/40/EC and for which the requirements laid down in UNECE-R 93 apply. Different deadlines – 2019 and 2021 – apply when it comes to implementing the current requirements regarding rear underride guards as per UNECE-R 58-03 in the context of vehicle approval.
The rear underride guard is a typical example of the continuous development of vehicle safety systems. First, new measures are proposed and negotiated. The result is often a compromise that is required to prove itself in real-life traffic situations. An integral part of the job of accident researchers to assess the effectiveness of such measures and, if necessary, suggest ways in which not only the vehicle design but also test speciffications could be improved. Nowadays, it is generally accepted that the rear underride guard fitted to a truck must offer at the very least adequate resistance in the event that a medium-size car collides with the rear of the truck with a dferential speed of 56km/h. This means that the car’s front crumple zones and restraint systems will function as desired, thereby protecting the occupants. Another measure necessary for promoting compatibility is sufficient protection of the car’s occupants, whereby requirements are to be based at least on the specifications laid down in UNECE-R 94 (front collision at 56km/h against a stationary barrier). At higher speeds, automatic emergency braking systems could help to reduce the kinetic energy to the greatest possible extent even before the collision occurs.
The performance of front underride guards as per UNECE-R 93 is generally accepted as sufficient, which is also due to the fact that the design and geometry of the front of a heavy-duty truck are largely standardized and harmless. Much less standardized and harmless are the design and geometry of the rear, especially in the case of trailers with a long rear overhang. It will therefore never be possible to completely rule out the risk of fatal underride accidents, particularly those occurring at high speeds on highways.

Stricter side protection requirements

For front and rear underride guards within the scope of the EU approval procedure and in accordance with UNECE-R 58 and UNECE-R 93, still no requirements apply regarding controlled energy absorption. Calculations and crash tests performed over the past few decades have shown time and again that controlled energy absorption not only reduces the stress peaks of the mechanical structures, but also provides a valuable, additional path of deceleration for restraining the car passengers. The US standard FMVSS 223 serves as an exemplary model of “best practice” here. The deformation work recorded during the static stressing of individual test points is calculated using a force-path line and compared with a predefined minimum value.
According to the applicable European specifications, front and rear underride guards fitted on heavy-duty trucks (classes N2 and N3) or trailers (classes O3 and O4) are designed to provide colliding vehicles (classes M1 [car] and N1 [light-duty trucks up to 3.5t]) with suficient underride protection. Much less stringent mechanical requirements apply to side protection systems, which have been prescribed in Germany since 1992 in accordance with Section 32c of the road traffic licensing regulations. Directive No. 89/97/EEC and UNECE-R 73 stipulate that side protection systems are to provide pedestrians, cyclists and motorcyclists with effective protection against falling under the side of a truck and being run over by the truck’s wheels. In the approval test, a side protection system is considered suitable if it can withstand a force of 1kN applied at right angles at any point from the outside.
In addition to mechanical resilience, geometric requirements regarding underride guards and side protection systems also apply. A key parameter here is ground clearance. The ground clearance for the front underride guard must be no more than 400mm, while the ground clearance for the rear underride guard must be between 450mm and 500mm, depending on vehicle-specific circumstances. For the side protection system, a maximum ground clearance of 300mm is required. Accident researchers have repeatedly called for more stringent requirements regarding side protection systems, and the issue is currently also on the agenda of the European Transport Safety Council (ETSC). In its position paper – published in March 2015 – on the revision of General Safety Regulation 2009/661, the ETSC demands, among other things, improvements in the strength of side protection systems to provide more effective protection against colliding motorcycles.

Retroreflective systems oncommercial vehicles

Many truck accidents occur during poor weather, at twilight and in the dark. One reason for this is that trucks are often not easily identificable as slower- moving vehicles, with the resulting risk that vehicles approaching a truck from behind might collide with its rear. In response to this hazard, internationally standardized specifications for specially identifying long, heavy-duty trucks and their trailers using retroreffective markings have been in place for some years now. This measure makes trucks more easily identifiable through “contour markings” consisting of retroreflective film, which mark out the vehicle contour to the side and rear. This means that trucks can be spotted from a greater distance, which is particularly important if they have come to a halt (or are lying on their side) on the road following an accident, when their own active lights are unlikely to be working. Contour markings on trucks help to improve road safety by allowing drivers of vehicles behind to better judge how far away the truck is and how fast it is traveling.
Retroreflective red-and-white warning markings (safety signs) are also widespread on vehicles used in the construction, maintenance and cleaning of roads or systems installed on or next to roads and are used in addition to, for example, rotating beacons. Police cars, fire engines, rescue vehicles and breakdown trucks are fitted with not only blue or yellow beacons but also specific, retroreflective markings to make them more easily visible particularly at night and, during the day, to provide extra contrast.