Basic Theory: Can You Spot the Error?

[JQ01]

The stopping distance from 48km/h is about 30m (6 - 7 car lengths), according to the tables on the next page (p74).  The reaction distance alone is about 13m and the braking distance is about 17m.  

 

The 2 second rule is not "how long does it take to stop including a 0.2s reaction time."  You just made that up.  

 

My understanding is that this so called "safe following distance", is the total stopping distance (inclusive of reaction distance).

 

Maybe my maths fail, but for 48km/h, with a stopping distance of ~13-15m, the time needed is around 2sec, not 1sec. So to me, this does not contradict the 2-sec rule.

[Fitvip]

Of course.  But the official guide for learner drivers states clearly that a 1 second gap is a safe following distance and then immediately afterwards contradicts itself by insisting that anything less than a 2 second gap might be unsafe.

 

As to your car-length standard - come on - it's between 4 and 5m for the average car - and certainly not 9 - 10m average!

Even the stopping distance at certain speed depends on many factors: if we take the driver's alertness and response time as the same; the others like the brake effectiveness, tyre rolling distance, dry or wet tarmac etc contribute to the actual stopping distance! A driver cannot be calculating all these while driving. Lay man say "gar agar". That is good enough.

 

As whether there is an error or not, by posting it here, is just seeking each individual's opinion, counter opinion and it may just end up one arguing with the other! 

 

Best way is to go to the source and trash it out once and for all, and, please do let us know, as many are very curious too!

[Slowpoke]

1 car length is what car?  Suzuki Swift or Mercedes S Class?

Edited March 7, 2018 by Slowpoke

[Sam7770]

1 car length is what car?  Suzuki Swift or Mercedes S Class?

 

 

Let's assume it is 4.5m on average.  (A typical car will be between 4 and 5m).

Where did u see the 1 second rule? 

 

It says 1car length for every 16km/h on the 1st paragraph and not 1 second.

 

That follows from their recommended car lengths, if we assume an average car length between 4 and 5m.

 

e.g.  48 km/h = 13.3 metres per second = 3 car lengths per second.

 

Diagram shows 3 car lengths as the recommended following distance at 48 km/h, therefore they are recommending a 1 second gap.

 

Let me know if there is anything else that is still unclear.

[Sam7770]

Even the stopping distance at certain speed depends on many factors: if we take the driver's alertness and response time as the same; the others like the brake effectiveness, tyre rolling distance, dry or wet tarmac etc contribute to the actual stopping distance! A driver cannot be calculating all these while driving. Lay man say "gar agar". That is good enough.

 

As whether there is an error or not, by posting it here, is just seeking each individual's opinion, counter opinion and it may just end up one arguing with the other! 

 

Best way is to go to the source and trash it out once and for all, and, please do let us know, as many are very curious too!

 

This seems like an ideal topic for a car forum.  Yes, we can have opinions about safe following distances.  But surely the official document has to be self-consistent?  I find it amusing the lengths people are going to to defend it.

 

I am hoping someone with better connections than I will put this query to the TP - I'm just a humble forum poster. 

[Daniu82]

Before passing, everything follows the book. After passing, the book is thrown out of the window and anyhow drive already. That's why so many crazy drivers on the roads mah...

[JQ01]

Let's assume it is 4.5m on average.  (A typical car will be between 4 and 5m).

 

That follows from their recommended car lengths, if we assume an average car length between 4 and 5m.

 

e.g.  48 km/h = 13.3 metres per second = 3 car lengths per second.

 

Diagram shows 3 car lengths as the recommended following distance at 48 km/h, therefore they are recommending a 1 second gap.

 

Let me know if there is anything else that is still unclear.

 

I will repeat my point one last time, not to argue just to put my point across clearly enough.

 

The paragraph 134 in the title "safe following distance" explained that "To be able to stop with an appropriate space...", clearly indicated that this "safe following distance" take into account that car need to brake and stop just in time.

 

48km/h is 13.3m/s, adding braking, at the end point, the speed should be 0m/s, so the time to travel 13.5m is ~2sec, so that gap is a 2sec gap, not 1sec.

 

I explain my case clear enough from my point of view. It will be better if you can share more if you got a reply from TP. 

[Soya]

Basic Theory got it all wrong.

 

They shld update their syllabus to reflect current SG driving norms. For example:

 

(1) Do not use your signal coz it will spoil easily (how else do u explain the tick tick sound, maciam timebomb gonna explode)

 

(2) Do not move off the lights quickly coz your engine will spoil easily

 

(3) Do not slow down and give way coz your brakes will spoil easily

 

(4) Do not be courteous on the road coz your fragile ego will spoil easily

[DACH]

Basic Theory got it all wrong.

 

They shld update their syllabus to reflect current SG driving norms. For example:

 

(1) Do not use your signal coz it will spoil easily (how else do u explain the tick tick sound, maciam timebomb gonna explode)

 

(2) Do not move off the lights quickly coz your engine will spoil easily

 

(3) Do not slow down and give way coz your brakes will spoil easily

 

(4) Do not be courteous on the road coz your fragile ego will spoil easily

Agree that driving schools should start teaching and stress on new drivers basic correct driving etiquettes and behavior.

[Sam7770]

I will repeat my point one last time, not to argue just to put my point across clearly enough.

 

The paragraph 134 in the title "safe following distance" explained that "To be able to stop with an appropriate space...", clearly indicated that this "safe following distance" take into account that car need to brake and stop just in time.

 

48km/h is 13.3m/s, adding braking, at the end point, the speed should be 0m/s, so the time to travel 13.5m is ~2sec, so that gap is a 2sec gap, not 1sec.

 

I explain my case clear enough from my point of view. It will be better if you can share more if you got a reply from TP. 

 

And I suppose I have to repeat that the typical stopping distance from 48 km/h is 30m, not the 13.5m you have made up.  Stopping distances are stated on the very next page.  The page we are discussing is about safe following distance at various steady speeds.

[JQ01]

And I suppose I have to repeat that the typical stopping distance from 48 km/h is 30m, not the 13.5m you have made up. Stopping distances are stated on the very next page. The page we are discussing is about safe following distance at various steady speeds.

I see. I did not make up. I count using the below formula...

 

DISTANCE = (InitialSpeed+FinalSpeed)/2 X TIME

[Sam7770]

I see. I did not make up. I count using the below formula...

 

DISTANCE = (InitialSpeed+FinalSpeed)/2 X TIME

 

How did you derive the distance to stop from 48 km/h?   Actual stopping distances are 2x what you state (30m vs 13.5m at 48 km/h).  If you're not using the official typical stopping distance, I can only assume you are making it up.

[JQ01]

How did you derive the distance to stop from 48 km/h?   Actual stopping distances are 2x what you state (30m vs 13.5m at 48 km/h).  If you're not using the official typical stopping distance, I can only assume you are making it up.

 

Alright. Here are my numbers:

 

Initial speed = 48km/h = 13.33m/s

Final speed = 0km/h = 0m/s

Time = 2sec

 

Distance = (13.3+0)/2 X2 = 13.3m

 

That's my calculation, not making anything up. Actually, I don't get your 30m stopping distance, the next page only says about 2-sec rule, it did not mentioned 30m.

[Sam7770]

Alright. Here are my numbers:

 

Initial speed = 48km/h = 13.33m/s

Final speed = 0km/h = 0m/s

Time = 2sec

 

Distance = (13.3+0)/2 X2 = 13.3m

 

That's my calculation, not making anything up. Actually, I don't get your 30m stopping distance, the next page only says about 2-sec rule, it did not mentioned 30m.

 

There you go:

[JQ01]

There you go:

 

Thanks. So indeed, my formula failed.

 

Taking the 50km/h example:

 

50km/h = 13.89m/s

 

Distance = 18m

 

time = (18 x 2) / (13.89+0) = 2.59sec

 

more than 2sec is needed...

[Sam7770]

Thanks. So indeed, my formula failed.

 

Taking the 50km/h example:

 

50km/h = 13.89m/s

 

Distance = 18m

 

time = (18 x 2) / (13.89+0) = 2.59sec

 

more than 2sec is needed...

 

I think you are calculating the distance you'd have to be from a wall when you start braking not to hit the wall! 

 

Two things -

1) the cars ahead also have to stop - so they continue to move out of your way until they stop.  (Unless they have hit the proverbial wall.)

2) braking distance increases with the square of speed.

Edited March 7, 2018 by Sam7770

[Nzy]

Alright. Here are my numbers:

 

Initial speed = 48km/h = 13.33m/s

Final speed = 0km/h = 0m/s

Time = 2sec

 

Distance = (13.3+0)/2 X2 = 13.3m

 

That's my calculation, not making anything up. Actually, I don't get your 30m stopping distance, the next page only says about 2-sec rule, it did not mentioned 30m.

 

I think the guy that wrote the book used the same formula  as you to derive the safe distance of 13.33m at 48km/h. He didn't realise that the 2s gap is at a constant speed since that 2s is for the reaction time.

[Sam7770]

I think the guy that wrote the book used the same formula  as you to derive the safe distance of 13.33m at 48km/h. He didn't realise that the 2s gap is at a constant speed since that 2s is for the reaction time.

 

I also think they may have looked at the diagrams they'd have to produce for a 2s gap and said, "Nobody drives at that distance - halve it - it won't fit on the page!".  Or perhaps they are stating that the gaps shown are the absolute minimum following distances (as I mentioned in an earlier post) - in which case they should make that clear, and not label it "safe". 

 

Wikipedia explains the 2 second rule being equivalent to 1 car length per 8 km/h not the 1 car length per 16 km/h recommended in the Basic Theory:

 

 

 

The two-second rule is useful as it can be applied to any speed. It is equivalent to one vehicle-length for every 8 km/h (5 mph) of the current speed, but drivers can find it difficult to estimate the correct distance from the car in front, let alone to remember the stopping distances that are required for a given speed, or to compute the linear equation on the fly. The two-second rule gets around these problems, and provides a simple and common-sense way of improving road safety.

 

Which further supports the point I'm making.