Of course it is possible to start arguing about technicalities, but if someone wants to cite mitigating circumstances I suspect the last thing on their mind will be playing the barrack-room lawyer and trying to trip up the system. A mitigating circumstance is a personal circumstance that overtakes a person. It is all very well to suggest that they could, if they knew it, take advice or argue whether the signs were in the right place, but I doubt whether that is what would be on their minds at the time.