There are two different entrances for the N12 each going in different directions. They are accessible by less than 20 minutes of walking from the center of town. Just follow the signs.
At the on-ramp of the motorway, the waiting time varies between 30 and 90 minutes if it's raining. If you have courage, do not hesitate to go to a car in a fire stop and ask – freely and simply.
East towards Rennes
The on-ramp of the motorway is at the end of Avenue Corneille. There is a traffic light and also a small place where people can stop just after it.
You can hitchhike from the roundabout between Boulevard Sévigné and Boulevard Harel de la Noé (it's called Rond-Point de Toupin) or some meters behind it before the bridge where you can find a small parking opportunity.
The best is probably the roundabout at the entrance of the D700 with a sign depending where you want to go. You can try it via Loudéac for Vannes or Lorient, and via Quintin for Quimper and Carhaix (but these you can also try via Guingamp in the northwest). Get a map and have fun on the small roads of Brittany.