A portable signpost assembly having a portable base structure and a frangible support post coupled to the portable base structure. The frangible support post includes a plurality of corners, wherein at least one of the corners, when viewed in cross section, includes a notch formed therein that promotes localized fracturing of the frangible support post upon contact from a motor vehicle. This enables the frangible support post to be broken away from the portable base structure upon impact from an errant vehicle to permit the sign and support post to travel over the vehicle without entering the passenger compartment.