The two-wheelers are definitely in the majority on the streets in India. Like dare-devils they zip between cars and busses, only millimeters away from sure death. Sometimes you see families of up to 5 people all fit on one bike.
Until recently only the driver needed to wear a helmet. I am not sure it was specified as being a motor helmet, because you often see people driving wearing a construction helmet... Also the size of the helmet is often laughable and the chin strap is just for show.
In January 2016 a new law came into practice; when riding on a two-wheeler you need to wear a helmet.; driver as well as any passengers. If you are being caught without, you will be fined up to INR1000 (13-15 Euro/USD)!
But, there is one exception to this rule; If you are a male Sikh, wearing a turban for religious reasons you are exempt from wearing a helmet... I guess they don't need protective headwear because the Sikh God will look after them...???