"Say I'm driving, see a 'bright sky' but no plume etc, no noise, just the sky brightening, how close would the probable detonation be?"(JP)
Depends on the actual type and size of the bomb and if it was detonated on, or in, the ground or in the air, as well as the terrain you are in. I'd say you were far enough away to safely turn the car around and hit the gas!
Small bomb maybe 5 miles? The biggest bomb, maybe 100 miles? I've heard the flash can be blinding if you are really close.
Thank you for the FYI, much appreciated