It is very safe, and there are many people who choose this method of getting around town. However, please be aware that street traffic is vehicle heavy, and nobody (and everybody) has the right of way. If you aren’t super confident on a bike, I suggest taking a taxi or walking through town. There are no crosswalks in Jaco or traffic lights, so be careful (and fast) when crossing the street. Traffic laws are not heavily enforced in Costa Rica, so make sure you look both ways before crossing the street.