I have no way of knowing how accurate this is, but here goes:
In order to build tubes, Earthworkers must first dig out a space for the tubes. Presumably, a small part of the tube is built above ground and then reburied after construction is completed, resulting in a small hill indicating where the tubes are below the surface. These hills act as a kind of speed bump (or hump, depending on where you live) forcing vehicles to slow down when passing over them.
Maybe.
Edit:
Alternate Theory: Because tubes are made of METTLE, the vehicles (which are also made of METTLE) must drive slowly over the tubes or the METTLE in the tubes will react with the METTLE in the vehicle, causing your colony to explode.