"how much distance is required before the driver comes to a complete stop? "
once the driver steps on the brakes, the skid marks on the pavement will start as well, therefore is really just asking, how long is the skid mark, namely what is "d".
[tex]\bf S=\sqrt{21d}\qquad \boxed{S=60}\qquad 60=\sqrt{21d}\implies 60^2=21d
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\cfrac{60^2}{21}=d\implies \cfrac{3600}{21}=d\implies \cfrac{1200}{7}=d\implies 171.42857\approx d[/tex]
other factors that can affect it will be, wind speed and wind direction, because the wind is acting on the car, atmospheric pressure as well, angle of the road, is not so easy to stop going downhill on a step hill, as opposed to a flat road. Other factors could be brake types, brake wear and tear, amount of weight in the car, if there are 5 sumo wrestlers in the car is not so easy to stop quickly.
