Buses in San Juan 

The bus system is really cheap in San Juan.  You can get bus route maps from your hotel, tourist offices or at ati.pr.  A ride is 75c but contrary to reports by Frommers, gotopuertoico.com, a multitude of other tourism websites and even the bus schedules posted at stops, buses do not necessarily run every 15-20 minutes; the average time between buses is more like an hour, and even on that schedule, the buses in San Juan can be extremely unpredictable. Additionally, traffic gets very heavy heading into Old San Juan on the T3 (from Sagrado Corazón train station), T5 (from Isla Verde), T21 (from Condado) and D53 (from the airport) so even catching a bus does not guarantee a quick trip. 

However, there are no convenient buses connecting San Juan to the rest of the island.  There are some públicos which go to various places on the island and are available at Río Piedras bus terminal.  You may want to ask around before you use them -  it is rumored that they stop everywhere and it takes ages to get anywhere.

Also have in mind if you are planning to get to the airport by bus, some bus drivers will not let you board the bus with more than one carry-on baggage (it could be to force you to pay the (relatively) expensive cabs).  If you have a problem, you can report it to the supervisors (who may or may not be helpful) or to the police.  It's less of an issue if you are starting from the airport.

Most routes work Monday through Friday from 5:00AM to 9:00PM; and Saturdays and holidays from 6:00AM to 8:00PM.

T3 and E40 express route are the only two routes that offers service on Sundays. The T3 route works everyday from 5:00AM to 11:00PM; and the E40 (from Piñero train station to Luis Muñoz Marín International Airport) from 5:00AM to 8:00PM.

For train and buses time tables you can download Tren Urbano App (unofficial app) available at Apple and Google Play Store.

 

San Juan public transportation map