Travelers are generally able to drive internationally with the International driving Licence (subject of the laws of the country you are travelling to) and usually both your home country licence and the international one must be presented to any police check points when you are overseas.
Sri Lanka-Transport-Driving: Contact the driving authority in the country you are planning to visit to find out their driver licence requirements. Please, do not drive without knowing the local laws.
Official website and the Authority in Sri Lanka:http://www.motortraffic.gov.lk
Sri Lanka’s train system is notoriously slow, but it is a good and cheap way to travel from city-to-city if you’re not in a rush. Even first class fares are reasonably priced (a good and much more comfortable option if you have an overnight or all-day journey). Otherwise the inter-city bus network is a fine option, too. The Sri Lankan Railways website offers a handy trip planner
Sri Lanka-Transport-Driving & Law
Cars can be driven in Sri Lanka with an international driving permit; however driving by yourself as a traveller here is not recommended, as the law is complicated.
If an accident occurs and the victim is hospitalised, the driver of the offending vehicle can be held in jail until the injured person is discharged from the hospital.
It’s best not to take the risk, and instead take advantage of the cheap and easily accessible railway and bus services or rent a car with a driver.
Sri lanka has oped few major high ways resonantly. I have used one of it is highway from Colombo to the south of the country. And well safe to driven.