[RspLtd]

Rrecently I attended a Speed awareness course. During the course we were shown a video which compared car firstly being driven at 30mph and the same car/driver/surface etc being driven at 31mph. At a given signal the car was brought to an emergency stop. The video insisted that the car being driven at the higher speed would pass the stop point of the lower speed car at a speed of 8 mph and travel considerably further. I questioned the maths and was slapped down by the trainer for being dispurptive and was threatened with being deemed to not have passed the course. Some quick maths seems to suggest this supposed research is flawed to say the least but Ihave been unable to find any published research which would either confirm or refute the alleged conclusions. Anyone any thoughts?

[Beelzebub]

Originally Posted by RspLtd

Except of course the Highway Code includes thinking distances.
30 mph has a total distance of 9m + 14m = 75 feet in English
40mph has a total of 12m + 24 m = 118 feet in English
Extrapolation assuming it is a linear change gives 0.3m additional thinking time and 1m additional stopping time. . So the 31mph car goes a further 4 feet to stop. Extrapolating from the Highway code it takes 40 feet to stop from 10 mph . As Spock would say - does not compute.

You're quite right. I had overlooked the thinking distance - it seems my schoolboy physics (or more likely my brain) was a bit rusty..

I have now recalculated. The assumed acceleration is actually considerably greater, at -0.66G, or 21.2 ft/s/s.

For the original question, between 30 and 31mph it makes no real difference - shorter distance but greater acceleration means the terminal speed at 31 is still about 7.8mph.

The effect is more spectacular in stopping from 7.8 mph, where the thinking distance alone is about 8 feet and overall stopping distance is just over 12 feet. Or maybe in the OP's words, "considerably further".

[ElCapitain]

I did one a few years back. The trainer just wanted to get through it and so did I.

[Beelzebub]

Originally Posted by RspLtd

Rrecently I attended a Speed awareness course. During the course we were shown a video which compared car firstly being driven at 30mph and the same car/driver/surface etc being driven at 31mph. At a given signal the car was brought to an emergency stop. The video insisted that the car being driven at the higher speed would pass the stop point of the lower speed car at a speed of 8 mph and travel considerably further. I questioned the maths and was slapped down by the trainer for being dispurptive and was threatened with being deemed to not have passed the course. Some quick maths seems to suggest this supposed research is flawed to say the least but Ihave been unable to find any published research which would either confirm or refute the alleged conclusions. Anyone any thoughts?

I too attended a course a few weeks ago. I'd seen the video before, and didn't question the maths.

I have now dusted off my 60-year old physics knowledge. Using the Highway Code stopping distance of 75 feet, and the equation v2=u2 +2as, gives an acceleration of -0.4G and a velocity for the second vehicle at 75 feet of 7.8 mph, or nearly 8mph

Sorry!

[Drivingforfun]

I’d have thought you could stop from 8mph in a matter of inches, I’d not call it considerably further

[Beelzebub]

Originally Posted by Drivingforfun

I’d have thought you could stop from 8mph in a matter of inches, I’d not call it considerably further

I make it 5 feet 4 inches.

[RspLtd]

Originally Posted by Beelzebub

I too attended a course a few weeks ago. I'd seen the video before, and didn't question the maths.

I have now dusted off my 60-year old physics knowledge. Using the Highway Code stopping distance of 75 feet, and the equation v2=u2 +2as, gives an acceleration of -0.4G and a velocity for the second vehicle at 75 feet of 7.8 mph, or nearly 8mph

Sorry!

Except of course the Highway Code includes thinking distances.
30 mph has a total distance of 9m + 14m = 75 feet in English
40mph has a total of 12m + 24 m = 118 feet in English
Extrapolation assuming it is a linear change gives 0.3m additional thinking time and 1m additional stopping time. . So the 31mph car goes a further 4 feet to stop. Extrapolating from the Highway code it takes 40 feet to stop from 10 mph . As Spock would say - does not compute.

[Rolebama]

My brain tells me that extrapolating the Highway Code figures would indicate the stopping distance from 10mph would be 15ft. Bearing in mind that sometimes my train of thought leaves the station without me.

Also, I remember reading in a medical publication, not that many years ago, that thinking times, (reflexes), are getting longer on average due to more people leading sedentary lifestyles.

[RspLtd]

@Beelzebub For want of being picky, the assumption was made and stated that the deacceleration was the same.

[Beelzebub]

Originally Posted by RspLtd

@Beelzebub For want of being picky, the assumption was made and stated that the deacceleration was the same.

Yes, and my calculations assume exactly that - the same value for both the 30 and the 31
mph vehicle.

My first (incorrect) attempt calculated that value as 0.4G, the later one 0.66G. A bit of research confirms that was indeed the figure used for the HC stopping distances.