This awesome vintage video first explains that bus companies are switching from gas to dirty diesels because they are cheaper. The best place to blow the exhaust out is through a low pipe because it leaves a nice concentrated cloud of soot on the ground. A demonstration shows that stacks blow it all over the road and the air current behind the bus mixes it all around. It also explains how a stack is more expensive and can be damaged, costing the company money.
This is the same situation today. School buses and motorcoaches have pipes near the ground so that the pipe doesn’t have to travel up inside. Whoever is behind one gets a nasty cloud of concentrated exhaust in their face while people to the sides breathe clean air. Big rigs use stacks because they have bigger dirtier engines so the pollution is made to spread out over a bigger area!
