Peter J. Weller
29 August 2011

A special type of collision problem occurs when a sedan sized passenger vehicle collides with the rear of a commercial sized truck having a bed 30 inches or more off the ground.  The problem occurs because of the height mismatch between the parts of the passenger vehicle that are designed to withstand collision (the bumper and engine compartment) and the rear face of the truck bed.  The result of this mismatch can be that the passenger vehicle slides under the bed of the truck, thereby allowing the bed to collide with the passenger compartment of the passenger vehicle.  This part of the car (the windshield and A pillars, if the car is colliding headon) is not designed to withstand frontal impact, and passenger compartment intrusion (PCI) by the truck bed may occur.  Passengers can then be injured If the truck bed penetrates far enough to contact the head or neck.  Decapitation sometimes occurs. 

Means of protecting the passengers from PCI have been proposed and studied for 50 years.  Generally the concepts take the form of a structural guard which is attached to the rear of the truck and extends below the bed to catch the car and limit underride.  These are called Underride Guards (URG).  Federal regulations in the US were first issued in 1953 (49 CFR 393.86).  These regulations stated the design requirements for the URG and required all trucks, trailers, and semitrailers with a Gross Vehicle Weight Rating of 10,000 lb or more and manufactured in 1953 or later to have URG.  Subsequent regulations which went into effect in 1998 (49 CFR 571.223) changed the design and performance requirements for URG, but only for trailers and semitrailers with GVWR over 10,000 lb.  The URG regulations for so-called “straight trucks” (ie, unarticulated trucks, or trucks that are not tractors in combination with trailers or semitrailers) promulgated under 49CFR 393.86 remained effective for those trucks only.  The US Department of Transportation has been criticized for not including straight trucks in the stricter regulations.  One of the reasons given for not including them is that, although straight trucks make up a large portion of the total heavy truck population in the US, they are involved in relatively few rear end collision accidents compared to trailers and semitrailers.


The dimensional requirements for URG for straight trucks (49 CFR 393.86 part (b)(1) are:

(i)  The distance from the bottom of the guard to the ground must be 30 inches or less.
(ii)  Separation between a two-piece URG must be 24 inches or less.
(iii)  The outer faces of the guard must be 18 inches or less from the outer face of the truck body.
(iv)   The guard must be 24 inches or less from the back face of the truck body.

Special purpose trucks constitute a subclass of straight trucks that may be exempt from all URG regulations, if they meet certain configuration requirements.  The basic requirement for classifying a truck as one of these special purpose trucks that do not need URG is that some part of the truck performs as a URG.  To do this, the structure must conform to the dimensional requirements for URG stated in section (b)(1) of 49CFR 393.86.  It is interesting to note that there is also a requirement for URG to be “substantially constructed and attached…” under section (b)(2) of this regulation, but the language exempting special purpose trucks (section (b)(3)) does not refer to this “substantial construction” section – only to the dimensional requirements.   
Examples of special purpose trucks with equipment that (depending on the design) can satisfy the dimensional requirements for URG include stakebeds or box trucks with lift gates, and car carriers with a tiltframe and attached light housing/foot , in addition to the stinger and crossarm. 

References

1.  49 CFR 393:  “Parts and Accessories for Safe Operation”, Code of Federal Regulations
2.  49 CFR 571.223 and .224:  “Rear Impact Guards” and “Rear Impact Protection”, respectively, Code of Federal Regulations
3.  Federal Register Vol 61 No. 16, Wednesday, January 24, 1996:  “49CFR part 571”.

Rear Underride Collision Protection For Special Purpose Straight Trucks