On cliffs you would have to be mindful of others walking at the bottom of them. My preferred place is Beachy Head, between the Red and White lighthouse and Belle Tout Lighthouse. My avatar roughly shows the place I will choose. Very few people walk down there and at high tide, there is no means to get back up the cliffs, so another point to consider.
I suppose it depends on whether you want to make the least impact on others. Off a road bridge could cause carnage for other people and would be very distressing for them. A bridge over water could be the better option.