RIYADH — At least 21 passengers suffered moderate to severe injuries Saturday, when two buses crashed into a truck, which was parked on the side of Qassim-Riyadh Road. One of the buses caught fire on impact. Civil Defense firefighters and Red Crescent paramedics arrived at the scene shortly after the accident had been reported. Some of the seriously injured passengers were airlifted to nearby hospitals. The two buses were coming from Abu Dhabi and were on their way to Makkah. Traffic officers started an investigation into the cause of the accident and why the bus drivers failed to see the parked truck on the road.