Generally, Barreirinhas is a fairly safe location for solo female travelers. It's a tranquil and welcoming small town that primarily serves as a gateway to the Parque Nacional dos Lençóis Maranhenses. It's quite tourist-friendly, and most of the locals are accustomed to visitors. However, like any other place, it's importantly to stay alert, especially at night, and avoid poorly lit or isolated areas. Use common sense and take standard travel precautions. Always make sure to inform someone about your plans, especially when going on excursions.