(Police is about to stop a car at a checkpoint(= 止められそうになっている車), or the speaker thinks "Police must stop this car at a checkpoint, since it's illegally converted" just by seeing the car.)
With the proviso below, I would read 大きな地震でもあれば崩れそうな崖 as meaning "cliff that looks like it would collapse if there was a big earthquake".Is that a valid interpretation?
However, on an unrelated point, I am not clear why でも is used here. To me it seems to have the wrong emphasis. Following でも, I am expecting it to say that the cliff would not collapse even in a big earthquake.