In a traffic jam, why don't people just start driving on the shoulder? Fear of getting a ticket?? What's the chance they'll pass a cop, who happens to be in the left-most lane, who happens to have sufficient time and room to even think he has a chance of chasing them down? A lot lower than the chances of getting caught speeding, which drivers often decide to do.

Same question, more universal situation: Often a lane leading to highway interchange is backed up, while all other lanes are running smoothly. Every driver reading this has at one point or another sped through a smooth lane, right up to the point of interchange, where they slow down and wait for some sucker to let them in. Why doesn't everyone do this, all the time? Why is anyone waiting in the slow lane?

Any rational incentive at play?