Overtaking with speed

[James]

Hi Soren good to see you posting again.

Good work with the diagram. One question though, surely variables such as 1) The performance of one's vehicle and 2) The reaction of the overtakee will have some play with the figures?

I am just thinking that slower vehicles may need more than the 0.3 of a second to gain sufficient momentum from the overtaking position to pass, having levelled with the speed of the traget vehicle. This is as opposed to approaching already at a speed sufficient to glide past...

Your thoughts?

[ScoobyChris]

I may have misunderstood this (it's not the first time), but if I perform a momentum overtake of a car travelling at 60mph and my speed is 1000mph surely my time exposed to danger should be less than if I perform the same manouevre at 100mph?!

Chris

[James]

"Sort of" my thinking...

[Søren]

Hi Soren good to see you posting again.

Good work with the diagram. One question though, surely variables such as 1) The performance of one's vehicle and 2) The reaction of the overtakee will have some play with the figures?

1)The performance is adjustable, just play with the acceleration and braking factors on the input table.
2) I was working on the HC understanding that the overtakee should do nothing other than maintain a steady speed while someone is overtaking (HC 144)

I am just thinking that slower vehicles may need more than the 0.3 of a second to gain sufficient momentum from the overtaking position to pass, having levelled with the speed of the traget vehicle. This is as opposed to approaching already at a speed sufficient to glide past...

Your thoughts?

Not sure where you get the 0.3 second from James. The comparison readings are total time readings incorporating acceleration to the point of no return in the case of the classic overtake, and constant speed to the point of no return in the case of the momentum overtake, then a deceleration to a defined safe point behind the overtakee should the overtake need to be aborted. I've perhaps made the model a tad more complicated (realistic) than was first realised.

[Søren]

I may have misunderstood this (it's not the first time), but if I perform a momentum overtake of a car travelling at 60mph and my speed is 1000mph surely my time exposed to danger should be less than if I perform the same manouevre at 100mph?!

Chris

Hi Chris,

I've adjusted the workings to take better account of the higher figures you were using. http://mysite.orange.co.uk/ksi/TED_new.xls

But you'll still see that a 1000 mph approach to a momentum overtake does leave much more time and distance exposed to danger than a 100 mph overtake. This is the point that 7db was alluding to earlier in the thread.

Why?

Because the point at which you commit yourself to an overtake at 1000 mph is a huge distance behind the overtakee; any closer and you will not be able to brake behind the overtakee if you needed to abort. As soon as you go beyond the point of committment you are in the Time Exposed to Danger, which is obviously a significant time and distance at 1000mph, and significantly less at 100mph (all variable depending on the braking efficiencies used).

[7db]

Nicely put, Soren.

I think the key point is to realise how hard is it to brake when you are catching someone quickly and they brake. mostly we don't get experience of this. When it happens, it is surprising how hard you need to brake.

You are committed very early at speed.

[ScoobyChris]

Thanks for clearing that up - makes much more sense now

Chris

[vonhosen]

Nicely put, Soren.

I think the key point is to realise how hard is it to brake when you are catching someone quickly and they brake. mostly we don't get experience of this. When it happens, it is surprising how hard you need to brake.

You are committed very early at speed.

But if you are already offside when they brake (& don't have to return to the nearside) it reduces TED, the problem is if your only option is to pull in behind them & match speed (or stop as the case may be) & you haven't anticipated that level of braking sufficiently early.

[James]

Hence watching the target vehicle - head movements / line movements...

But more importantly thinking for them as they approach hazards that come late to them but early to you...

[Søren]

Nicely put, Soren.

I think the key point is to realise how hard is it to brake when you are catching someone quickly and they brake. mostly we don't get experience of this. When it happens, it is surprising how hard you need to brake.

You are committed very early at speed.

But if you are already offside when they brake (& don't have to return to the nearside) it reduces TED, the problem is if your only option is to pull in behind them & match speed (or stop as the case may be) & you haven't anticipated that level of braking sufficiently early.

The model adopted a simplistic consideration of the scenario of overtakee braking, on the basis that this was not always a threat to the overtake, and I assumed that to brake without colliding with the overtakee, it was firstly essential to be able to brake to match his speed (with a inputtable safety margin) without colliding with him.

The main danger consideration in my mind when constructing the model was the concept of aborting the overtake for whatever reason, eg a hazard appears within the overtaking arc. In this case (and in the simplistic overtakee braking model) my thinking was that TED commenced at the point beyond which you could safely brake and return to your side of the road without coming too close to the overtakee.

[7db]

The definition of the zone of danger in my original post was where you couldn't stop behind the target until you were clear in front.

Obviously if you can slip by then you get away with it. Similarly if you have a jetpack and can jump him, or a nice wide easy right appears so that you can abandon the road altogether.

Either way you are committed to avoiding him very early at high speeds should he decide to lock up and slide sideways into the middle of the road at the sight of a bunny appearing from nowhere.

[Søren]

Either way you are committed to avoiding him very early at high speeds should he decide to lock up and slide sideways into the middle of the road at the sight of a bunny appearing from nowhere.

I see that as one of the dangers you are exposed to while overtaking.

[Luke]

and my speed is 1000mph

Chris

Grief, what car do you drive Chris? [/i]

[7db]

Either way you are committed to avoiding him very early at high speeds should he decide to lock up and slide sideways into the middle of the road at the sight of a bunny appearing from nowhere.

I see that as one of the dangers you are exposed to while overtaking.

Yup - and the one that you are minimising in your technique.

[Horse]

I don't wish to upset our engineering/scientific brethren, but I agree with Luke. You don't learn to drive a car by figuring out all the maths behind it. An overtake is a matter of judgement and spatial awareness. If it were all down to numbers, why aren't all the best racing drivers, bowls, snooker and darts players engineers?

And YES, I hate numbers. My brain works very well with words, but numbers just do not flow smoothly. It's like trying to write with the other hand... possible but messy. Something to do with being left handed and the right side of the brain perhaps?

As I read this (as in the spreadsheet & comparisons, etc), the point seems to be that the sums show how different the two 'styles' of overtake are in the way that the overtaker has to commit earlier, and may need to react differently, rather than there being an intention that all drivers should whip out a calculator before overtaking and that mental arithmetic should be in the driving test

It might seem that catching someone else up, and maintaining speed for the evertake would be the 'easiest' way to overtake, perhaps lulling the overtaking driver in to a false sense of stupidity, while in fact that method has its own set of 'gotchas' to plan for.