In this case we couldn't see any external injuries so suspected it merely had a concussion, which it would recover from no worries given time. The problem was just that we couldn't intervene to help it, and it was in the middle of the road. The real threat to it at that point was cars. Even moving it off the side of the road and into the grass would have probably been enough.
In such a situation we normally pick up the bird, pop it into a dark cardboard box where it is stimulus-free, and let it recover for a little while. Generally when you open up the box later the bird is alert and ready for release.
It went against all our instincts to drive away and leave it in such a vulnerable state.
