The information on this page is taken from costaricalaw.com website.

People seems to forget very basic condition: your tourist visa and your drivers license are valid for 90 days, not 3 months. If your passport expires before the 90 days of your entry date, you will be denied the entry. If you stayed over 90 days without renewing your visa, your foreign drivers license will be considered expired and you will be prosecuted for driving without a license, if you decide to drive.

Driving in Costa Rica is not impossible, you just have to adopt your driving to the other drivers on the road and the geographical layout of the roads. People who drove on mountainous roads in there home country will not have any problems, but must keep in mind that you will find many drivers that will go for passing in a most dangerous situations. In this case just slow down and try to avoid a collision, others will do the same.

Below are some fine examples:

Making an illegal “U” Turn                                                    ₡280,000
Driving Using Cell Phone                                                          ₡94,000
Speeding in Excess of 120 Km/Hr                                        ₡280,000
Speeding 20 km / hr above the speed limit                           ₡47,000
Speeding 30 km / hr above the speed limit                           ₡94,000
Speeding 40 km / hr above the speed limit                         ₡189,000
Speeding in a school zone above 25 km / hr                           ₡94,000
Driving Under the Influence of Alcohol 0.020 to .050        ₡280,000
Running a Red Traffic Light                                                    ₡189,000
Failure to Buckle the seat belt                                                  ₡94,000
Not having the vehicle inspection up to date                          ₡17,000
Foreigners driving without a local license after 3 months    ₡47,000
Driving with an expired vehicle registration                          ₡47,000
Illegal Parking                                                                             ₡47,000

Some Spanish expressions you will need to know when driving in Costa Rica:

Tarjeta de Circulación  – Vehicle Registration Card
Marchamo  – Vehicle Registration Sticker (posted on windshield)
Riteve  – Vehicle Emissions and Inspection Certificate
Traspaso de Vehiculo  – Vehicle Title Transfer
Oficial de Transito  – Transit Police
Infracción – Transit Violation
Boleta de Infracción – Traffic Ticket
Señal de Alto – Stop Sign
Carretera – Highway
Calle – Street
Seguro – Insurance
Accidente de Transito – Traffic Accident
Velocidad – Speed
Zona de Descarga – Unloading Zone

All information above is taken from another website and the accuracy is not verified.