As a cyclist my primary concern is diesel trucks with scant emissions controls belching particulate and other combustion byproducts. I see little movement to get these off the road.
For anyone else curious, I googled it and it looks like a delete-pipe, (aka muffler delete, cat delete) is basicly a muffler/catalytic converter bypass device. Some do it for performance, some for the noise, some explicitly to "piss off tree-hugging liberals".
Oh yeah, we get plenty of them out here in the rural parts. Friggin stupid.
We've got a little '81 tractor that's a 3-cyc 1.4L diesel and if the wind blows the wrong way you definitely don't want to inhale too deeply. I'd love to convert it to Tier IV but there just aren't kits made for it.
I've seen one person riding with something that looked like a gas mask. It appeared to have a "bane" like square filter near the mouth. Not just a simple N95 mask.