> Are diesel vehicles actually more environmentally friendly?

Yes (legal) diesel passenger vehicles and light trucks sold in the US since [edit: 2007] are significantly cleaner in all respects than their gasoline equivalents, although that difference goes away soon as the gas regulations catch up.

It's very unlikely you could detect whether the vehicle in front of you is gas or diesel just by looking at it or smelling the exhaust.

That said, there are a lot of pre-2005s out there, showing smoke and smelling bad. Also the heavy trucks are just now getting the emissions requirements that lighter vehicles have had for the last 10 years.

I have a 2012 heavy-duty diesel pickup. Except for the logo on it and maybe the sound of the engine, you'd never know it was diesel.