Speed decides two things: how long you need to stop, and how badly people are hurt if you cannot. A speed limit is a maximum, not a target. The right speed is the one at which you can stop within the distance you can see to be clear, on that road, in that weather, in that traffic.
Regulations 16 and 17 of the Motor Vehicles (Driving) Regulations, 2017 require drivers to keep within the limit and to leave enough space to stop safely behind the vehicle ahead. National maximum speeds for each type of vehicle and road are set by MoRTH notification S.O. 1522(E) of 2018, but the limit posted on the road always applies and is often lower. Indian law gives no single figure for following distance, so this guide teaches the three-second gap as a practical rule, and more when visibility or grip is poor.