Which ever you find easier. Find a fixed point ahead i.e. a telegraph pole. As the car in front passes it, in good driving conditions, say to yourself at normal speed, "Only a fool breaks the two second rule!" If you passed the pole before you finished it, you are too close to the vehicle in front. If you finished it way before you reached the pole, you are too far away from the vehicle in front. The two second rule mantra automatically adjusts the distance between vehicles dependent on speed. However, you must increase the distance by at least 50% for wet/slippy conditions. Repeating the first half of the phrase will do that.
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It is generally recommended to measure the distance in seconds rather than feet or car length when following another vehicle. This is because measuring in seconds provides a more accurate representation of the time it takes to react and stop in case of sudden braking. It also allows for a consistent measurement regardless of the speed of the vehicles involved.
I would use a centimetre to measure the length of a boy. Not sure what a boy yard is, though.
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You could measure the length or width of it but you would have a decimal. You should be using centimeters.
You can't convert between units that measure completely different things. In this case, length and time.
The length of your foot should but not on everyone. The average person's foot measures up.