Horgos-Röszke border crossing
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It is situated on the E75, which is a motorway to Budapest and an express carriageway to Novi Sad. There is heavy traffic, both trucks and cars. Hungary is part of the Schengen Agreement but Serbia isn't, so border controls can take some time. Leave the truck and cross the border on foot. On the Serbian side it happened to pite twice that he was sent away by the police to the petrol station further down the road.
A lot of Serbian people living next to the border cross here frequently for some, well, unknown reasons. They may not be the most cooperative people in terms of taking hitchhikers. Also the police likes to control people for cigarettes and other stuff, that's why it can be tricky to get a ride crossing the border. As it is possible to cross the border by walking, just convince the drivers to leave you nearby the border crossing and walk. Anyway it will be much faster than the whole procedure with a car. On Hungarian side walk for 300 meters and you will find a petrol station where you can ask drivers. Be aware that most of the cars are Serbian drivers going somewhere nearby, so it is not that easy to get away, but you will not starve there either.