Coimbra is a city in Portugal.
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.
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.