If you are driving down a residential street at 60 KPH and a child chases a ball into the road without looking for cars, will tragedy result?  To answer this, you need to consider many factors such as the condition and weight of the car, the road surface and the type of braking system in the car. But there are two human factors that must also be considered.  First, when does the child run out into the street and how much distance does this leave between you and a terrible accident?  Second, what condition are you in as the driver?  Are you alert and focussed on your driving? Or are you fatigued?

If you are alert and focussed and the child runs into the street more than 42 metres in front of you, it will likely end well.  But any shorter and it might end terribly.  The 42 metre stopping distance has two parts.  From the time you see the child and your brain processes the information, and you extend your foot to the brake pedal and then flex your muscles to slam on the brakes, you will travel 24 metres. This is referred to as the reaction distance. It will then take your car another 18 metres, the braking distance, to stop (24 + 18 = 42 metres).

Here's an interesting video that shows how the total stopping distance increases exponentially with increases in speed:

Now what happens if you are fatigued? This is where things go bad.  Fatigue slows down your reaction time[1]. This means that the 24 metre reaction distance will increase.  And if you are the type of person who panics easily or your brain reacts as if the child running in front of you is an emergency situation, your reaction time can "block". A "block" is a significantly longer reaction time[2]; it is similar to freezing in the face of danger.  A reaction time "block" can occur in response to an emergency or a situation where it is difficult to make a decision[3].

So what happens to the 42 metres at 60 KPH? If you "block" and your reaction time doubles, this could increase the overall stopping distance to 66 metres (24 x 2 + 18 = 66 metres)!

If you are planning a summer road trip, read these tips before you go.

References

[1] See for examples:

(A) Babkoff, H., Mikulincer, M., Caspy, T., Kempinski, D., & Sing, H. (1988). The topology of performance curves during 72 hours of sleep loss: A memory and search task. Quarterly Journal of Experimental Psychology, 324, 737-756.
(B) Fiorica, V., Higgins, E., Iampietro, P., Lategola, M., & Davis, A. (1968). Physiological responses of man during sleep deprivation. Journal of Applied Physiology, 24(2), 169-175.
(C) Linde, L. & Bergstrom, M. (1992). The effect of one night without sleep on problem-solving and immediate recall. Psychological Research, 54(2), 127-136.

[2] See for examples:

(A) Bills, A. (1935). Fatigue, oscillation, and blocks. Journal of Experimental Psychology, 18(5), 562-573.
(B) Shinar, D., Zaidel, D., & Paarlberg, W. (1978). Driver performance and individual differences in attention and information processing. Volume 1: Driver inattention, Report No. DOT-HS-8-01819-78-DAP, NTIS No. PB 292165 & 6.

[3] See for examples:

(A) Shinar, D., Zaidel, D., & Paarlberg, W. (1978). Driver performance and individual differences in attention and information processing. Volume 1: Driver inattention, Report No. DOT-HS-8-01819-78-DAP, NTIS No. PB 292165 & 6.
(B) Teichner, W. (1968). Response blocking. A necessary performance criterion. Pre-crash Factors in Traffic Safety. American Association for Automotive Medicine, Proceedings of the12th Annual Symposium, 165-169.