Humans are different from other animals in the extent that they can use mental simulation to provide pseudo-feedback for themselves (“I wonder what would happen if I tried to climb over that razor wire?”)

Pigeon spikes turn out to be discomfort based rather than pain based. The effective ones are not sharp. They are flexible. The pigeon cannot roost properly. They try to hold onto the spike, but it falls over under their weight. There is no position where they can roost without a spike interfering with wings or feet because they are too densely placed to avoid.

Not sure about cattle grids but I can definitely say that electric fences are a learned obstacle. On our farm, when I was growing up, when we wanted to train a new horse that an electric fence should be avoided, we would place food on one side and the horse on the other. A nasty, but memorable, shock resulted when the horse went for the food, and from then on it took only a single strand of wire to keep half a ton of horse under control – it didn’t even need to be switched on all the time.

Greg Smithsimon writes, “Prickly space technology has the advantages of being easily retrofitted onto existing spaces, and of representing an unambiguous message of control to potential users.”

But are they always so unambiguous?

Influence or control can be exercised by social or physical means. Consider the relative merits of a “no loitering” sign, regular police/security patrols to move loiterers on and the simple absence of anywhere (comfortable) to sit.

In my neighbourhood, there are large areas of the footway with raised, irregular cobblestones that are clearly designed to say “don’t walk here” and to channel pedestrians away from their natural desire lines onto routes that the designers considered to be safer. Yet they seem to be largely ineffective, the stones neither reading unambiguously as “no walking” nor sufficiently prickly to prevent walking rather than to make it more difficult (and ironically, more risky).

I suspect the intention of these cobbles is observed more often by locals who are familiar with the area and will also have learned the “correct” routes to take rather than visitors to whom their intention is unclear and for whom the alternatives are less obvious. A study of who walks on the cobbles, who doesn’t, and why, might be enlightening.

Other examples spring to mind: cattle grids and electrified fences on farms.