Energy goes up with the square of velocity. A car going 140 MPH has 4x the energy of a car going 70. Assuming a 3,000 lb car, there was about as much energy in the car as 1.4 lb (.64 kg) of TNT, but applied in a single direction, much more efficiently than an explosion would. Modern cars are impressively safe, but there probably wasn't much left.

Edit: He was driving an SUV, ~5000 lb, so closer to 2.4 lb (1.1 kg) of TNT. It's a miracle that anybody survived.

There's an engineering saying: "Any idiot can build a bridge that holds a load. An engineer can build a bridge that barely carries a load." It looks like they overbuilt the bridge to compensate for lack of solid engineering.