Convert speed units between units, using exact SI factors.
Divide kilometres per hour by 3.6 to get metres per second: 90 km/h is 25 m/s. The 3.6 comes from 3,600 seconds in an hour divided by 1,000 metres in a kilometre. The SI unit of speed is the metre per second, which is what physics calculations use.
A knot is one nautical mile per hour, or 1.852 km/h. Shipping and aviation use it because it maps directly onto coordinates: one knot is one minute of arc per hour. The BIPM SI Brochure accepts the knot for use alongside SI.
Sixty miles per hour is 96.56 km/h. One mile is 1.609344 kilometres, a ratio that follows from the exact definition of the inch in SI units. So a US road sign reading 65 mph is about 105 km/h.
Because kinetic energy is proportional to the square of speed. Double the speed and braking distance roughly quadruples rather than doubles. In SI units it is plain: energy is ½mv², and that square is what makes speeding so dangerous.