I think you may want to think twice about driving in Sri Lanka given the following information from the Foreign Office. I would also suggest you read their whole page on Sri Lanka (travellers now need a visa to visit this country),
"Many roads, particularly outside the major towns are in a serious state of disrepair which prevents being able to travel at speed or comfort. But there is considerable investment in the redevelopment of the road infrastructure throughout Sri Lanka. You need an International Driving Permit and a Sri Lankan recognition permit to drive in Sri Lanka. You can obtain a recognition permit at the AA in Colombo. A British licence or an international licence alone will not suffice.
Erratic driving is common and as a result, road accidents are frequent. Pedestrians and animals often appear in the road without warning. Vehicles do not stop at pedestrian crossings. Always wear a seatbelt. Be particularly careful when overtaking and when overtaking vehicles are coming towards you. If you drive, make sure you are comprehensively insured. Driving at night outside Colombo and driving by motorbike are particularly hazardous. If you have a collision, stay at the site of the accident with your vehicle as long as it is safe to do so. If it is not safe or if you feel threatened, report to the local police station.
Security checkpoints and roadblocks are common throughout Sri Lanka. Pay attention when passing through them, and always obey the instructions of the police/army on duty. In particular, stop if asked to do so. There have been cases where security forces have opened fire on vehicles that do not stop when asked. Roads around Government and military sites in Colombo are regularly closed for security reasons (e.g. for VIP convoys), so you should allow extra time for your journey if passing through these areas.
Buses are notorious for driving fast and rarely giving way. They are often poorly maintained. A number of serious bus accidents have been reported over recent years.
Taxis and hire cars are inexpensive in Colombo and tourist areas. Motorised rickshaws (tuk-tuks) are readily available for hire in towns and villages. Agree a price before you travel. Most drivers are respectable, although there have been reports of harassment, particularly of lone female travellers at night. You should exercise caution and change to a different rickshaw if you have any concerns about the driver or standard of driving"