This is because of several factors, mostly having to do with cost. The obverse of this is that since the builder must bear the cost, any person or neighborhood with enough money can put one up if they want to. When I see two topes within twenty yards of each other on the outskirts of a dusty village with nary a school in sight, I imagine some bizarre family feud ending in the erection of rival monuments, four inches high, six inches wide, and thirty feet long. Most topes are either concrete or bituminous in composition. The concrete ones tend to be thinner than their bituminous brothers, with squarer corners.
Topes come in several basic models.
There is the common single strand tope, described above. There is the double strand, and occasionally the triple strand. After that there are vibradores . Vibradores are composed of several low, thin topes, with very small gaps in between. Much like the warning strips that US motorists encounter before they come to a stop sign on the highway, they differ in their severity.