Dense fog claimed seven lives when an unidentified heavy vehicle hit two cars on the Chandigarh-Ambala highway in Haryana early on Saturday, police said.The victims belonged to Chandigarh and included three women, two minors and their driver.Three others were injured in the accident and rushed the Civil Hospital in Ambala before being referred to the Post-Graduate Institute of Medical Education and Research (PGIMER) in Chandigarh as their condition was serious.The accident took place in the fog-enveloped highway close to the Drive In-22 resort near Ambala, around 40 km from here.The victims, who were in two cars, were on their way from Chandigarh to Vrindavan in Uttar Pradesh. They had stopped on the highway as one of the cars developed a snag and the heavy vehicle rammed into their cars from behind in the heavy fog.Police said the driver of the unidentified heavy vehicle escaped from the spot.Eight persons had died in a massive pile-up of vehicles in thick fog in Haryana's Jhajjar district on Monday.Dense fog has been prevailing over most parts of Punjab and Haryana in the past few days, especially along highways. The region is experiencing cold wave conditions for over a week.