Highway Code Stopping Distances Observation/Puzzle

[Swervin_Mervin]

Prompted by a discussion elsewhere about the unrepresentative nature of the HC's stopping distances guidance I had a look at them (I've probably not looked at them since I was 17!) and was intrigued to compare them to the Stopping Sight Distance (SSD) requirements that are laid out in design guidance.

Anyway, looking specifically at the HC figures, I'm convinced I must have something wrong here:



Taking the 30mph/64kph example, the reaction time equates to 0.67s (works out the same reaction time for 40mph as well) - 13.4m/s at 0.67s = 9m. However, the bit that surprised me is the actual stopping distance of 14m for 30mph. By my maths that works out at a deceleration rate of 12.8m/s2. Which seems neck breaking to me!

For reference, modern design guidance determines SSDs for cars on the basis of a deceleration rate of 4.5m/s2. And that's based on research.

Anyway, end of boring post

[Sundayjumper]

Highway Code distances in feet were always quite obviously set by a formula:

mph x 1 = thinking distance
mph^2 / 20 = breaking distance

Code: Select all

speed	think	break	total
10	10	5	15
20	20	20	40
30	30	45	75
40	40	80	120
50	50	125	175
60	60	180	240
70	70	245	315
80	80	320	400
90	90	405	495
100	100	500	600

It was always quoted in feet. Your table above has got some kind of rounding error @ 40mph presumably by someone converting feet to metres and back to feet again but you can see the others line up perfectly

[Swervin_Mervin]

Interesting!

Looking at this more, the HC is based on braking at 1.3g (at 30mph). That's a lot! I know it's supposed to account for emergency scenarios where the anchors are slammed well and truly on, but even so. Standards work on 0.45g (for cars) and that's still to account for people stopping at the sight of a hazard.

Anyone have a g-meter app that they fancy testing?

[Jobbo]

When I learnt to drive a mate of mine pointed out that the stopping distance (in feet) is speed (in mph) squared divided by 20, and the reaction distance in feet was just the speed in mph. So I always remembered d = v^2/20 + v - I learned the distances but it was a useful double check that I'd remembered correctly. The reaction time at any speed is always the same: 0.66 seconds.

The relevant equation of motion for the acceleration you undergo would be v^2 = u^2 + 2as (see https://www.ncl.ac.uk/webtemplate/ask-a ... otion.html ). Got to use SI units, so:

start speed (v): 30mph is 13.4m/s
distance (s): 75ft is 13.7m
end speed (u) is just zero

So with a zero end speed, a = v^2/2s gives 6.55 m/s2 which is about 2/3G. I think maybe you've misremembered your equations of motion.

[mik]

HC numbers haven’t changed for nearly 50yrs AFAIK.

It’s been tested by many - modern cars stop in 50-60% of the stopping distance quoted by the HC, and hopefully most people will manage to react in less than the glacial 0.67s also.

It’s now so far from reality that it’s of no help to anyone. Like setting a section of motorway to be - 20mph limit, or marking the height limit on a bridge at 1.8m when it’s quite clear that it’s more like 3m (height kills).

[mik]

Anyone have a g-meter app that they fancy testing?

Nope, but I did inherit a 1960-ish brake tester from my dad. Fully working (rarer) clamp-on model with gimbal. I believe these generally clamped onto the running boards (what what), and they pre-dated current rollers which test the efficiency of each wheel separately.

Only worth about £120, but it’s a cool object.

[mik]

Might be worth a bit more, but I can't see myself selling it.

One went for £275 at a Bonhams auction - I reckon mine would clean up similarly. That one didn't have the leather case.

https://www.bonhams.com/auction/24879/l ... ey-and-co/

Note: when I go on holiday I put a D-Lock through this, so don't be getting any ideas anyone.....

[Swervin_Mervin]

When I learnt to drive a mate of mine pointed out that the stopping distance (in feet) is speed (in mph) squared divided by 20, and the reaction distance in feet was just the speed in mph. So I always remembered d = v^2/20 + v - I learned the distances but it was a useful double check that I'd remembered correctly. The reaction time at any speed is always the same: 0.66 seconds.

The relevant equation of motion for the acceleration you undergo would be v^2 = u^2 + 2as (see https://www.ncl.ac.uk/webtemplate/ask-a ... otion.html ). Got to use SI units, so:

start speed (v): 30mph is 13.4m/s
distance (s): 75ft is 13.7m
end speed (u) is just zero

So with a zero end speed, a = v^2/2s gives 6.55 m/s2 which is about 2/3G. I think maybe you've misremembered your equations of motion.

I knew I'd gone wrong somewhere. I'd actually calculated the time to stop incorrectly and then worked from that time. That would make a lot more sense at 6.5m/s2!

For info the standards work on a perception/reaction time of 1.5s and a deceleration rate of 0.45g (4.4m/s2). They also factor in gradient, however. And the decel rate can be adjusted with lower figures for HGVs and buses also. So a 30mph SSD is tyipcally 43m for cars/LGVs. That's a lot more than the 23m the HC states.

[Swervin_Mervin]

Anyone have a g-meter app that they fancy testing?

Nope, but I did inherit a 1960-ish brake tester from my dad. Fully working (rarer) clamp-on model with gimbal. I believe these generally clamped onto the running boards (what what), and they pre-dated current rollers which test the efficiency of each wheel separately.

Only worth about £120, but it’s a cool object.

That is very cool mik

[Rich B]

Note: when I go on holiday I put a D-Lock through this, so don't be getting any ideas anyone.....

[Sundayjumper]

I did something this morning that you could have put a d-lock through.

[Beany]

I did something this morning that you could have put a d-lock through.

You stole a bike? You monster.

[jamcg]

I did something this morning that you could have put a d-lock through.

Parked up a ///M2?

[mik]

WRT the above - posted a Q on boatshoes and someone found this vid from 1928 (no sound), so it might be older than I thought.