Exeter

Like anywhere in the southwest, you can pretty much hitch anywhere within reason and expect a wait of no more than 15 minutes. This is ofcourse assuming you look clean, sane and friendly... also there needs to be a reasonable flow of traffic and since Devon has the most milage of road compared to any other County, this could mean one car an hour is not uncommon on the single track lanes between villages.

Exeter is normally easy if you know where to go, take a look at a map for the green 'A' roads heading out to your respective destination and jsut head for the juncton you see on the map. Dual carriagways are not Motorways and so it is not technically illegal to hitch on one, however it is ill advised. If police or more likely some stuck up locals tell you to move, there is no legal reason to do so unless you are being a fool.

The M5 heading north is the only restriction to where you are legally allowed to hitch. However it starts quite early into the center of Exeter and there is pavements here and there that most people are happy to pull over onto even in the suburbs of exeter. My advice would be to either find a layby where you can hitch before the M5 or get a bus a little further to the next village/town and hitch on a slip road.