How fast can I drive?

This is a very common question asked by all drivers. How fast can I drive?

There you are driving along nicely and the road is clear. You wish to just hit the pedal and race away. Then you see this sign:

Figure 1 Speed limit sign 60 mph is 100 km/hr

Oh come on, you may say, why can’t I drive faster than that? Why are these silly speed limits put up here?

If you are thinking like that, or you really wish to know why there are speed limits, please read on till the very end and you will be glad you did.


Figure 2 Speed thrill and safety


How are speed limits set? Are they some random number posted by some officials in the Transport department, or are they potentially lifesaving?

To answer this question, let us look at two very important concepts here:

Stopping Distance: This is the time taken for a moving vehicle to come to a halt. You may be surprised to know that a car travelling at 120 kmph can sometimes take the length of a Football field to stop!

Reaction time: This is the time taken for the driver to register danger and hit the brakes, typically this take one second. In this one second, your car would have travelled 33 metres or over a hundred feet if you travel at 120 kmph!

Both of these are variables depending on the speed of the vehicle, condition of the road and tyres and braking system of the car, and alertness of the driver. It takes quite a bit of Math, Physics and Biology to calculate the speed limits for any stretch of road.

Reaction Time Components

1 Mental Processing Time

This is the time it takes for the responder to perceive that a signal has occurred and to decide upon a response. For example, it is the time required for a driver to detect that a pedestrian is walking across the roadway directly ahead and to decide that the brakes should be applied. 


2. Movement Time

Once a response is selected, the responder must perform the required muscle movement. For example, it takes time to lift the foot off the accelerator pedal, move it laterally to the brake and then to depress the pedal.

3 Device Response Time

Mechanical devices take time to engage, even after the responder has acted. For example, a driver stepping on the brake pedal does not stop the car immediately. Instead, the stopping is a function of physical forces, gravity and friction.

The intelligent driver will error on the safe side and leave room for reaction time and less than perfect conditions. That driver will also hone the braking skills to give more of a margin of safety. That margin can save lives. Pay attention to the need to react quickly.

Reaction time increases in poor visibility. Low contrast, peripheral viewing, bad weather, etc. slow response. 

One of the most difficult situations occurs when a driver must detect motion of the car immediately ahead, its acceleration or deceleration. Accidents frequently occur because the driver fails to notice that the car ahead has stopped and does not apply brakes until it is too late.

This happened a few days ago to none other than the Chief Minister’s convoy.

Figure 3 Ignoring speed limits, and it’s consequences, make the news almost daily

So speed limits are based on complex calculations to minimize the risk of accidents. A comprehensive study of road safety (Treat et al., 1977) found that human error was the sole cause in 57% of all accidents and was a contributing factor in over 90%. In contrast, only 2.4% were due solely to mechanical fault and 4.7% were caused only by environmental factors.

Figure 4 Car accident

If you ignore speed limits it would be fatal not only to you but to other innocent people as well.

So, please, even if you may have a Lamborghini, please follow the speed limits,because your life is far more valuable than any car ever made.


Here is some more technical stuff if you are interested:

Highway traffic and safety engineers have some general guidelines they have developed over the years and hold now as standards. As an example, if a street surface is dry, the average driver can safely decelerate an automobile or light truck with reasonably good tires at the rate of about 15 feet per second (fps). That is, a driver can slow down at this rate without anticipated probability that control of the vehicle will be lost in the process.


The measure of velocity is distance divided by time (fps), stated as feet per second. The measure of acceleration (or deceleration in this case) is feet per second per second. That assumes a reasonably good co-efficient of friction of about .75; better is .8 or higher while conditions or tire quality might yield a worse factor of .7 or lower.


No matter the velocity, that velocity is reduced 15 fps every second. If the initial velocity is 60 mph, 88 fps, after 1 second elapsed, the vehicle velocity would be 73 fps, after 2 seconds it would be 58 fps decreasing progressively thereafter. For the true mathematical perfectionist (one who carries PI to 1000 decimal places), it would have been technically correct to indicated the formula is ‘fpsps’ rather than ‘fps’, but far less understandable to most drivers. Since at speeds of 200 mph or less, the difference from one method to the other is in thousanths of seconds, our calculations in these examples are based on the simple fps calculations.


Given the previous set of conditions, it would mean that a driver could stop the described vehicle in a total of 6.87 seconds (including a 1 second delay for driver reaction) and your total stopping distance would be 302.28 feet, slightly more than a football field in length!

Published by Dr. S. P. Mathew

A Mumbai-based practising Physician, passionate about God's wonderful creation, fitness, and integral health, writer, activist and family man. Saved by God's Grace alone :-) to be his Ambassador.

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

%d bloggers like this: