Sumatra (alternative spelling Sumatera) is an island in western Indonesia. It has an area of 470,000 km² with 45 million people (as of 2005) living on it.
An anonymous hitchhiker says: Very easy hitching.
(2013) It can be easy to hitch around in Sumatra, provided you bring a lot of time and nerves. It's important to look for the right car - that would be a truck or pick-up truck. Chances to get a ride in a private car are pretty low, since there are not so many private cars driving around. You will notice a lot of more or less nice looking SUV's but about 99% of them are so called "Travel Cars". These cars are somewhat like a mixture between a taxi and a bus. Because the public transport in rural areas is very limited, people use travel cars to get from town to town. If you're stuck somewhere and are considering to take a travel car, don't forget to bargain for the price and before that, ask people around for the normal price ("harga biasa").
However, it's very easy to hitch a pick-up truck, they will most likely stop, if you wave at them (wave with the right hand as the left hand is considered as dirty) and they don't expect you to pay. If you want to hitch a big truck, better ask the drivers directly, if you see a truck stopping somewhere (they usually stop at gas stations or roadside restaurants for breaks).
Coming back to the first sentence, I will explain now: Even if you're lucky and don't have to wait long times for cars to stop, the road conditions are mostly very rough, so cars can't drive very fast. Therefore a journey about 200 km might take you a whole day. You need nerves, because whereever you stop, people will try to convince you taking a bus or travel car or whatever kind of transportation is available there. Just tell them you want to ride for free ("menumpang gratis").