No, that's not how fission reactors work. There would be a meltdown, yes, so there would be a small hole in the ground where the core melted, and the car burned into a cinder. But; no nuclear yeild. The bigger concern is when the billowing smoke goes into the air. Radioactive waste would drift halfway across the country everytime there was a serious car accident.

Imagine a country full of Mini-Chernobyls scattered everywhere across the nation.

Nope, not a good idea.

As a former nuclear engineer, I think it would be wise to build an adequate supply of new/highly reliable reactor plants for electrical power (less oil). Along with that, it would be VERY wise to keep nuclear power in one place at a power plant, not cruising down the highway at 70mph.