is deuterium really ssafe to be used as fuel for mobile vehicle? why is it not widely used?

It's expensive, and offers no performance benefit over hydrogen as a fuel for combustion. In fact, on a weight basis, it is only half as good. There are, however, no particular hazards associated with it. Deuterium is at present useful for making hydrogen bombs, and not much of anything else. In time, processes may be developed to use it as a nuclear fuel for power generation, but the problem has been worked on for fifty years and success is nowhere in sight.