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From Dejvická (subway line A) take bus 119 to Dědina. There you can hitchhike. You can also try another bus stop on Evropská road e.g. Divoká Šárka but probably further is better. Good spot but most cars going towards Kladno or Slaný.

This is better option for Karlovy Vary direction. Take tram 22 to Bílá Hora. Go straight in the street Karlovarská and there is quite good place to hitch-hike behind the red-white banister. '''Third Option to Karlovy Vary'''If you prefer gas stations, try this option. The first one is quite far though (30 km from Prague center) - take a bus from Praha,,Hradčanská to Velká Dobrá,,u školy. Once there follow the direction of the bus and turn first left (Unhošťská street), than second right (Lesní street) and after 200 m you should see the highway and the gas station on your right.