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Ankara is the capital of Turkey, surrounded by a big ring road.
Get dropped off at any exit on the ring road and you should be able to hitch to either a metro/bus stop or if you are lucky all the way to downtown.
Option A: Kızılay (city center), there is the main dolmuş and city-bus station located just at Güvenpark. Cross the pedestrian bridge over the huge avenue, and take the first street on your left called Meşrutiyet Caddesi which is full of bus stops. Find bus nr. 123 to Yaşamkent. The bus goes about 30 minutes and brings you to the starting point, where is a huge shoulder to stop plus a petrol station. Use a sign, you are just before the interjunction, where highway splits to Istanbul and other directions to the seaside.
Option B and C: Take the Metro to Eryaman 1-2.
B: Take bus 501-2 to Susuz Köyu (7 stops). From there 200m walking to the ramp (Istanbul motorway and the alternative road via Bolu). Many ppl got direct rides to Istanbul from here according to the Hitchwiki map.
.C: Bus 521 feom Kizilay to Ankata Çevre Yolu or with bus 531/541 from Eryaman 1-2. The spot isn't perfect in my opinion. Cars are fast, officially you can't walk onto the motorway, many hitchhikers do so though and are luckier than at the beginning of the ramp. I had to wait 2h. No clue what's the reason. Another guy seems to have had a direct ride to Istanbul within five minutes.
Northeast towards Samsun
Here you should actually just take a dolmuş going in direction of Kırıkkale; from there you can easily hitch-hike up north to Samsun. You can find such minibuses in Ulus in city centre, next to the Julian column. These minibuses will have a sign saying "Mamak", the name of the easternmost suburb of the city. They will take you as far as to the ring road surrounding the city for 2TL per person. Note that there are many dolmuş terminals around Ulus, so it's a good idea to ask which one has buses going to Mamak rather then just going to every single one.
Along the highway between Ankara and Kırıkkale, there is a gas station every few hundred metres (nope, not kidding). Waving with your hand and thumbing around these stations work fairly well, and trucks stop easier. You can also ask people at the gas station.
It is no exaggeration that there are gas stations every few hundred metres. When riding from Ulus to Mamak, don't be tempted to get off at the first bunch of gas stations you see. Instead, Try and get the driver to take you beyond the ring road where there are many rides going long distances. The closest town to the bunch of gas stations beyond the ring road is called "Ortakoy"... only 500 metres or so from the gas station although fence climbing might be necessary.
South towards Konya, Adana & Cappadocia
Take a metro or dolmus to the main bus station called A.S.T.I. From there you can already see the highway. If you turn right you can see the signs saying Konya and Golbasi. Just walk along the highway, which is only about 2 km. You can either thumb on the road or try your luck on the fuel station which is situated there (in January 2012 the staff was very helpful to rozwal in finding a ride). From there you can easily hitch to the ring of Ankara which has the junction in Golbasi, or you just take a Dolmus to Golbasi for 2 Lira. The dolmus stops at a crossover of the highway.
To get to Cappadocia Region, find a hitch to Aksaray and look out for the signs for the road to Nevşehir (just when you enter the city, there's also a gas station where your driver can stop). From there you can easily stand on the side of the street, many trucks going that direction, too.
Food recycling organization ÇerÇöp Çorbacılar (Trash Soup) operates a free food case on Karanfil Street in the city center, Kizilay. You can take what food you need, and if you somehow find yourself hitching out of Ankara with more food than you want to carry, you can leave your extra food there as well.
ÇerÇöp Çorbacılar has a Facebook page where you can ask questions in English.