It is strongly recommended to avoid a trip when it is dark. The combination of rickety roads condition and weak lighting are the main reasons for not to walk in complete darkness. It is very dangerous especially for a driver who has no skills to drive on this island.
Some roads, especially in Nuwara Eliya and Horton plains, it is required a high driving skills and experience.
For these reasons, the ideal way to travel in Sri Lanka for those whose holiday lasts up to three weeks is with a vehicle and a personal driver / guide. You can hire self-drive vehicles, but if your avoid driving by yourself is better because there are streams of bicycles, motorcycles, ox-carts, cars, buses and trucks on roads. As traffic laws are only a recommendation for roads’ safety, it is required a skilled driver who knows the roads well.