Coimbra is a city in Portugal.
North and Interior towards (A1) Aveiro, Porto, Braga; (A25) Guarda
Go walking (Coimbra is a small town, you don't need to take the bus) in the direction of "Avenida Fernão Magalhães" (http://maps.google.com/maps?hl=pt-BR&tab=wl from Hotel Astoria to Avenida Fernão Magalhães or from Santa Cruz Church to Avenida Fernão Magalhães). All the cars that are travelling direction north must go there. As there is a big amount of traffic there, they drive really slow and you can easily stay on the right side of the road.
South towards (A1) Lisboa, Évora, Faro
Go to the down town (Portagem) and cross the Santa Clara bridge. You can easily see the signs that say 'Lisboa A1'. Just follow it. You can put yourself on the right side of the road Avenida de Conimbriga and wait there for a hitch hike.
You might notice this squat-looking houses in Coimbra upper district, around the university. Republicas are not Social Centers nor regular flats neither, they are kind of common houses inhabited by students and are part of the culture of this city. Some of them are quite radical and many are amazingly antique, always worth the pity to knock the door and ask if you can know more about the history of this places, you may will be invited to stay for a night.