Delhi's smog is thicker this winter thanks mainly to the multiplying number of vehicles , a study by Tata Energy Research Institute (TERI) has noted. The number of vehicles in India is expected to touch the 43 million mark by the turn of the century-- more than double the number in 1900--with the largest vehicle population in the national capital. But Delhi had to cope with more than a fast growing vehicle population, the TERI report said. Use of poor quality fuel, substandard vehicle technology and congestion were the major problems.