Overtaking with speed

[Gromit37]

Horse:

This may come across as pedantic and pig headed: but it's not meant that way, honest!


As you come round the bend, hoping for a straight on which to overtake the slower car that you have been catching up with for the last 30 seconds... you simply look to see if the 'momentum' overtake can be performed in half the distance you can see to be clear. Assuming all other criteria are met, you go for it, or you slow and wait. I suspect that you don't decide between the two, you just discount one, leaving only the alternative. Your behaviour is governed by the choice you make, probably within a second or so of getting position to see the road ahead, so the actual maths is neither here nor there as long as you judge it to be safe. Spatial awareness gets my vote every time.

[7db]

The point being that having sat here in the quiet reflection with the maths beforehand, you'll have realised how early you are committing to the overtake, and you'll be able to train yourself appropriately.

Momentum overtakes on exits to blind corners just don't work for me - you need to be carrying far too much speed into the unknown. It's triangles all the time where corners are involved.

Did a lovely one last night in the dark on the A283, with great cooperation from the target who illuminated the road excellently and flicked main beams off at just the right time. Got a nice wave from me. Had to decline about five others which might have been on but couldn't get cooperation.

[Horse]

This may come across as pedantic and pig headed: but it's not meant that way, honest!

As you come round the bend, hoping for a straight on which to overtake the slower car that you have been catching up with for the last 30 seconds... you simply look to see if the 'momentum' overtake can be performed in half the distance you can see to be clear. Assuming all other criteria are met, you go for it, or you slow and wait. I suspect that you don't decide between the two, you just discount one, leaving only the alternative.

Actually, I think you've just emphasised the point I was making - you want to do the 'momentum' overtake because it's easier (and means you won't have to bother slowing down and go to the effort of 'planning' an overtake.

While the truth, according to the figures, seems to be that you expose yourself (and, presumably, other road users) to danger over a longer distance and more time!

I can do pedantic too

[VinnyP!]

What danger? I avoid it like the plague. that's why I am not a big fan of this term TED just time committed to the overtake. That graph correctly points out that time. But if you plan your drive the time exposed to danger is not increased. As I said before the momentum overtake will often be on when a triangle isn't since the gap in oncoming traffic can be that much smaller, since a momentum overtake means less time on the offside.

Momentum overtakes were definitely encouraged on the exit from bends on every course I've been on. I'd go so far as to say when hunting you'd be heavily criticised for not dropping off and closing through the bend so that if the view opens up you have a differential to take the maximum advantage any offside gap, hey if memory serves it's even in the bible.

[7db]

Vinny, I'd distinguish between a gentle close through the bend, moving into a triangle and a full momentum overtake.

I'd suggest that anyone carrying a 20mph differential into a blind bend with a car inside their stopping distance hasn't got a long-term strategy for collision avoidance.

[ScoobyChris]

and my speed is 1000mph

Chris

Grief, what car do you drive Chris?

A diesel Mondeo. Let's just say I have a vivid imagination

Chris

[Søren]

I've had a little play and tweak with my original model (it's now nearly 1 meg because I don't do calcularse ) http://mysite.orange.co.uk/ksi/TED_new.xls .

I've included 7db's definition of T.E.D. (or time committed to the overtake). The minimal liklihood of the overtakee braking to a standstill, or revealing somthing in front of him causing you to have to brake to a stop behind him is a valid inclusion in the total time.

It's interesting to see the difference this consideration actually makes to the time and distance to which you are committed to the overtake.

Here are a couple of charts which show the change in TED and DED for different approach speeds or attainable speeds. (the little wobbles in what should be smooth lines is as a result of the method of calculation used in lieu of calculus) In each case the overtake is on a 40 mph 15m vehicle by a 4 metre vehicle returning to your side of the road 20 metres beyond the overtaken vehicle.

TED


DED

Interestingly the speeds at which we would normally overtake with 'momentum' - 15 to 30 mph faster than the overtakee's speed - is the range which has the greatest relative difference in time exposed to danger favouring the 'classic' overtake.

[7db]

What does the speed along the x-axis indicate in the classic overtake? Aren't we assumed to be following at 40mph before going?

[Søren]

What does the speed along the x-axis indicate in the classic overtake? Aren't we assumed to be following at 40mph before going?

It's the maximum speed to be attained, ie accelerate up to that speed then constant speed after that. The flatline indicates that the speed to be attained had not been reached by the time the overtake was completed.

[7db]

And just to clarify - does the momentum overtake car just maintain speed - no acceleration?

[Gromit37]

The point being that having sat here in the quiet reflection with the maths beforehand, you'll have realised how early you are committing to the overtake, and you'll be able to train yourself appropriately.

Momentum overtakes on exits to blind corners just don't work for me - you need to be carrying far too much speed into the unknown. It's triangles all the time where corners are involved.

Did a lovely one last night in the dark on the A283, with great cooperation from the target who illuminated the road excellently and flicked main beams off at just the right time. Got a nice wave from me. Had to decline about five others which might have been on but couldn't get cooperation.

The maths doesn't help me overtake... I use my eyes and my brain to decide if there is room. Most drivers will never see the maths, but they do overtakes. Some of them even do it safely! I've seen the maths, but I won't be using it. Overtaking on "blind" bends isn't what I meant. You are catching up on a vehicle, there is a bend ahead. Put yourself in a position where you can choose to overtake or ease off depending on the view etc.

[Søren]

And just to clarify - does the momentum overtake car just maintain speed - no acceleration?

Yep.

[7db]

I've seen the maths, but I won't be using it.

Use it or not, you will still be living within the heavy dull leaden scope of its inescapable gaze.

[TripleS]

The point being that having sat here in the quiet reflection with the maths beforehand, you'll have realised how early you are committing to the overtake, and you'll be able to train yourself appropriately.

Momentum overtakes on exits to blind corners just don't work for me - you need to be carrying far too much speed into the unknown. It's triangles all the time where corners are involved.

Did a lovely one last night in the dark on the A283, with great cooperation from the target who illuminated the road excellently and flicked main beams off at just the right time. Got a nice wave from me. Had to decline about five others which might have been on but couldn't get cooperation.

The maths doesn't help me overtake... I use my eyes and my brain to decide if there is room. Most drivers will never see the maths, but they do overtakes. Some of them even do it safely! I've seen the maths, but I won't be using it. Overtaking on "blind" bends isn't what I meant. You are catching up on a vehicle, there is a bend ahead. Put yourself in a position where you can choose to overtake or ease off depending on the view etc.

Agreed. That's pretty much my feeling. Some may find the maths interesting, but ultimately it's your judgement on the road that governs these things. In fact, if one were to attempt to do calculations on site, so to speak, you'd surely find the overtaking opportunity had gone anyhow. It could be a dangerous distraction from the processes that need to be employed.

Best wishes all,
Dave.

[7db]

Strewth.

How would anyone suggest showing that their intuition or judgement is wrong?