No matter what. If the traffic lanes look bad before you clean, be sure to inform the customer that some traffic lanes will never look as good as the rest of the carpet due to the foot traffic etc.
It's better to let them know BEFORE you start cleaning so it looks like a reason instead of an excuse.
If the Traffic Lane DOES clean up nice....Then you've just become a hero.
Under promise, over deliver....but always be honest about your work and do your best to get it as clean as possible.
With all said.....What they said above me is some good info to get the job done right.