How practical would it be to use a giant microwave to scan underneath a vehicle?

We posed this question to Dr Bill Proud...

The vehicles are generally moving along the road at some speed, say 30 miles an hour. The article you're trying to cause to explode is buried underground so you have to make sure that the waves can actually reach into the explosive, and all of this has got to happen in the millisecond or so that you've got in front of the vehicle. So you need a huge amount of power and you can't mount that much power and drive it down the road, basically baking the road in front of you to the required temperature. Also, think about what would happen to the wildlife and the general ecology around us as we're sending out that much power.