The physics of a vehicular accident vary greatly based upon the vehicles involved. When two normal passenger vehicles are involved, the physics, while not simple, are well understood, and there is specific software used to run analysis. However, once the vehicles exceed those parameters, with size, load, and design requirements, the physics can become much more complicated. Heavy truck and heavy machinery vehicles are a specialization all their own.
The first question is of load. Not only does the load, in its weight, placement, and mobility, have a great effect on the outcome of an accident, what the transport is carrying can be vitally important as well. Certain types of loads bear more risk, such as sour crude, which can release deadly gas on an accident scene. This is one of the reasons that having Transport Canada certified engineers investigating the incident is of utmost importance; violations of the code can have far reaching consequences once an incident occurs.
There is also the value of the load to consider. A truck transporting a crane might roll or hit an overpass with little consequence to the truck, but may result in a complete write off of the crane and infrastructure damage to the roadway or overpass. This is another reason why a qualified mechanical engineer specializing in insurance and legal investigations is a primary concern when investigating such a case.
The regulatory landscape of transport vehicles is a specialization all its own. There are many considerations beyond the certification of the vehicle involved in code. For example, was it licensed at that time in that location? Regulations for highway transport are differ greatly from the rules for passenger vehicles.
And of course, as with any incident at speed, there is the question of forces at work. With the complicated linkages between trailers, the physics of a highway vehicle accident can become significantly more complicated than one involving regular passenger vehicles.
These are only some of the reasons that a specialized engineer is best when it comes to an incident involving heavy machinery, trucks, or other highway transport vehicles.