Barreirinhas is a city in the State of Maranhão in the North of Brazil and known as one of the main getaways (see more below!) to Parque Nacional dos Lençóis Maranhenses.
North towards Atins (Parque Nacional dos Lençóis Maranhenses)
Although hitchhiking from Barreirinhas to Atins technically can be done, it always requires a bit of talking and eventually quite some patience. While a few supply and worker's cars go from Barreirinhas to Atins every day, most of the time you will have to talk one of the guided tour jeeps into giving you a lift. This can be done best at the obligatory ferry crossing of Rio Preguiças in Barreirinhas (02°44′39.39″S 42°49′35.26″W) where everybody, including the guides and the people paying for the tour, will get off their jeeps for a few minutes. Without basic knowledge of Portuguese or Portunyol it won't get any easier, but still possible as long as you manage to explain yourself. Get creative, introduce yourself and what you are doing, offer the guides to ask their group for permission to take you and don't be shy! The tours leave Barreirinhas starting from 7.30am until approximately (not completely verified!) 11am. If you decide to spend the night in Atins, it may be a little more difficult to hitchhike/get back without paying as, apparently, most of the locals drive to Barreirinhas between 5am and 7.30am and usually charge anyone and especially tourists R$ 25.
In January 2019 vapaakulkuri and his partner got a ride with a tour jeep from Barreirinhas to Atins and later on even got another tour to take them around several beautiful lagoons within the national park. At this time seemingly all of the tour jeeps arriving to Atins first went to a restaurant known as "Cabana do Peixe". It is owned by a lovely family that will help you out with drinking water, a place to leave your backpack, a shower and a place to set up your tent. Please, if you happen to go there, would you leave Maria a hug from Dominic and Anna from Germany? Thank you!
East towards Jericoacoara, Fortaleza
To go East from Barreirinhas along the coast, there is a dirt motorway starting 2 km out of town at the large, yellow and blue Ipiranga gas station. The dirt road starts right beside it. It has free bathroom and showers if you need them. It may be better to use a sign for Tutóia first, then get dropped off and go for Parnaíba or further.
The dirt highway extends to the small town of Paulino Neves. There the paved MA-315 begins. If you get dropped off in Paulino Neves, there's a yellow petrol station just outside of town where you can catch all the traffic. You can use a sign for Tutóia or Parnaíba with luck. When coming into Parnaíba, asked to be dropped off where the current motorway (343) meets motorway 402. There you can walk a kilometer down to an orange petrol station to hitchhike and talk to drivers for rides going further toward Fortaleza.
Other destinations (Parque Nacional dos Lençóis Maranhenses)
You could also try hitchhiking from Paulino Neves towards Caburé and there try to have one of the boats give you a lift to Atins. You can also quite easily access the national park (by walking and hitchhiking) from Santo Amaro do Maranhão.