While trucks are gradually being brought up to passenger cars safety standards most of the ones now on the road lack such simple and necessary safety features as head rests and side impact protection. Perhaps most importantly, most trucks cannot meet the roof strength standard for cars, even though, as a class, they are more likely to be involved in rollover accidents than cars. There is a simple, no cost fix for this which the industry has failed to employ because of ignorance and lack of interest.
Diesel fuel is much harder to ignite than gasoline.
There is generally only one possible ignition source for a diesel fuel fire following a crash, gasoline can be ignited by a number of sources. For more about semi's see the Truck Safety page .