It is disgraceful that obvious Civil Engineers are pontificating over this issue. Unfortunately gentlemen this is an aero/me crossover problem and requires sophisticated analysis of adiabatic air flow principles as found in Ascher Shapiro advanced fundamnetals of air flow

This analysis revolves around the fundamentals of lift itself 1/2 pv2 A.

Analysis as follows: The tractor was traveling at 62.5 mph this basic calculation is the impulse of the tractor and its backhoe being absorbed by the friction coeficcient of steel on concrete a widely know factor of 20 slugs per inches per inch t.

Added factor as the backhoe progressed down the highway the inverted bucket caused down force on the tractor trailer lowering its height but more importantly causing the bucket itself to press downward. As the tractor approached the overpass the down force was blocked by the bridge causing the loss of airflow over the buchet(inverted) and thus lift not downforce was created causing the bucket to smash upward through the bottom of the bridge abutement. The delay in airflow is the reason why the bucket popped up AFTER the tractor was under the bridge missing the guardrails.

the author is a mechanical/aero major and a lawyer so this must be correct Dick Engel