Chris Galtress (asker) Dec 8, 2008:

celu-ci I am sure it relates to the bus. I going with "the bus must have a clean exhaust to..."

Emma Paulay Dec 8, 2008:

Quite! Yes, it does read like an afterthought.

Bourth (X) Dec 8, 2008:

Peculiar expression Can't help wondering it if was originally just "propre" and someone decided to add "à l'échappement" rather than rewrite it to make sure no one understood they were talking about vehicles that had been put through the buswash and thoroughly vacuumed.

Emma Paulay Dec 8, 2008:

Or does it refer to "le bus"?

Emma Paulay Dec 8, 2008:

Chris, what does "celui-ci" refer back to? "Propre à" probably has the sense of "particular to" here.