A head-on collision between two buses on the Oruro-Potosi highway in Bolivia has left at least 22 people dead and more than 30 others injured, police said.The accident occurred on Saturday on the highway around 40 km from the city of Challapata, Romulo Delgado, commander general of the police, told reporters.The injured have been taken to hospitals in Challapata and Oruro city. The incident was under investigation, Xinhua reported citing the police officer.According to preliminary police report, the accident could have been caused due to excessive speed and one of the buses may have crossed into the oncoming lane, in addition that rain and fog could have obstructed the drivers' vision.