I was thinking about the exhaust thing.
Now this is conjecture, I don't know the truth.
BUT, diesel exhaust pollutant is the particles of soot.
These are heavier than air and ARE nasty if ingested into our lungs.
If a truck blasts that exhaust upwrds then it will fall to ground and people are at risk from ALL of the soot.
If it's a side/rear then most of the soot will fall to the ground and it will be below our mouth/nose. Some WILL be turbulent and go higher to settle back down again.
It seems to me (engineer-mode-on ) that high-exit exhausts are a LARGER risk for air-breathing people than low-level. Unless of course you're in a low sportscar or a kiddie in a buggy
With modern diesel engines the manufacturers have been investing in attempts to remove these particles from the exhaust. Until they do so - and get all the old ratty trucks/busses off the road - I guess I prefer it exiting low. I admit that I will close the windows and air vents if I see a 'dirty' bus/truck ahead to avoid taking it in.