At least 15 passengers were killed in a road accident when their bus skidded off the road and overturned in the northern Indian state of Punjab on Friday, police said.The privately-owned bus, with more than 50 passengers on board, was travelling from Nawansheher town to Jalandhar, local police official Snehdeep Sharma said.Sharma said initial investigations suggest that the bus curved at a high speed and then lost control, skid, overturned and hit a tree on the Jalandhar-Chandigarh highway. More than 20 passengers were injured.According to government data, an average of 135,000 people die on India's roads each year.Most accidents are caused by poor conditions of roads and vehicles, poor driving or overcrowding of vehicles.