A car's kinetic energy is affected both by its weight and its speed. As long as the weight or speed of the car does not change, its kinetic energy does not change either.
If the car's weight changes (for example, by load or passengers) then the kinetic energy changes in proportion to the weight change. A doubling of the weight also doubles the kinetic energy.
If the car's speed changes, the kinetic energy changes quadratically in relation to the speed change. A speed increase from 30 km/h to 90 km/h will multiply the kinetic energy nine times (3 to the power of 3 is 9).
If the kinetic energy is multiplied by nine, braking distance is also multiplied by nine, as braking distance is directly proportional to kinetic energy.