Turning Right at a T-Junction

[lyndon]

I'm trying to check my intuition. Suppose you are stationary at a T Junction, about to turn right. Traffic is approaching from your left at an estimated 60 mph. How big a gap do you need, if you are not going to inconvenience an approaching driver at all, i.e. he can continue at 60mph and always remain a safe distance from you.

I'm assuming I can get across the road in zero time, and accelerate to 60 in 8 seconds. I may manage better, but that seems a nice relaxed assumption. In 8 seconds I reckon I will have travelled about 120 yds. But a car already travelling at 60 will cover 234 yards in that 8 seconds. I reckon if he is going to be at least 60 yards behind me after 8 seconds, he has to be 174 yards to the left of the junction when I start my maneuvre. OK, so I can't judge 175 yards, so maybe I should make it 200 yards to be safe.

Does that tie up with your intuition?

[IVORTHE DRIVER]

Morning,

I am assuming this post is put here to wind us up a little but hey its too good to ignore.

Intuition can be a dangerous thing unless you can back it with solid experience

Your maths are impressive but flawed.

0-60 in 8 seconds?, may well be do-able but on a dry perfect surface, and no way whilst turning as well, add to that white lines (no grip) man-hole covers (no grip) patched tarmac (no grip) etc etc...add at least another 10 secs

If your car is front wheel drive that kind of acceleration will most likely torque steer you straight into the field opposite!

If you have rear wheel drive that kind of acceleration when turning will see you so far sideways the chances are the car approaching at 60mph will t-bone your drivers door!

4 Wheel drive...at best a snapped drive shaft will result in you doing sweet f.a untill the oncoming car hits you.

Across the road in zero seconds? unless you are in Harry Potters night bus...not going to happen

Assuming you have passengers ask them to get out, not only will it lighten the car it also means you will only kill yourself ( and sadly the driver of the approaching car)

If you cannot estimate 175 yards what makes you think you can estimate 200?

Are you sure the other car is driving at 60 mph?

My suggestion...turn round and go home, might be safer for us all

Have a good day

Ivor

[lyndon]

Thanks for your help, Ivor. What I'd like to know is what people would consider to be the minimum distance for an approaching vehicle under these circumstances, and what they think their 0-60 time would be. Can you help with this?

[GJD]

Are you sure you need to think this analytically about it? I don't think I use the sort of time and distance calculations you're thinking about. Next time you're in this situation and you judge it's OK to pull out ahead of someone, you could look in your rear view mirror to see whether they catch up with you more quickly or less quickly than you expected. Either way, you can then adjust your judgement next time you're in a similar situation. Would that approach work for you do you think?

[gannet]

problem is it is SO subjective

minimum to one driver is different for others - even in the same car.

one has to make a judgement at the time...

[Ancient]

You cannot judge the approaching car's speed accurately enough to 'do the maths', nor (if you can't judge 175 yards) can you judge 200 yards accurately enough.

What you can judge IME is the time the car is taking to approach you, the number of seconds between cars and the number of seconds the car will take before it is 'on you'. That is also more useful as you can (should be able to) judge the time you'll require to get up to speed on this road surface, this turn in in these weather conditions with the hazards observed to be down the road.

Time is IMO a much better (more easily used) measure of the gap you need and the gap which exists.

[lyndon]

Are you sure you need to think this analytically about it?

Well, I've managed for 50 years without thinking this analytically about it, so I'm certain you are right. I just think it would be interesting to test my intuition. Some of our instincts are tested many times every time we go out driving - like stopping distance is reinforced every time you come to a stop.

I'm not proposing that drivers should think about things as analytically as this when actually driving. It's just a bit of armchair driving that I find useful as I drift further into senility.

[lyndon]

problem is it is SO subjective

minimum to one driver is different for others - even in the same car.

one has to make a judgement at the time...

I fully appreciate that. But for me, testing my judgement is important too. I agree with CJDs advice about checking the mirror to see what actually happened, but it's quite possible, for example, that every time I think I have done it right, the approaching driver had to at least ease off the throttle to give me space.

Bottom line is I know my judgements are too conservative. I am being encouraged to make more progress in many different ways, and this includes not being overcautious at T-junctions. But I'm not prepared to overrule my sense of what is safe. For me, an analytical approach may help me reduce the gap without jeapordising safety. Not that I intend doing the maths when sitting at the junction, of course.

[ScoobyChris]

Bottom line is I know my judgements are too conservative. I am being encouraged to make more progress in many different ways, and this includes not being overcautious at T-junctions. But I'm not prepared to overrule my sense of what is safe. For me, an analytical approach may help me reduce the gap without jeapordising safety. Not that I intend doing the maths when sitting at the junction, of course.

Not sure whether you're currently doing an advanced course or have completed one so apologies if I'm teaching you to suck eggs, but one way to re-evaluate/recalibrate your own judgements is to drive from the passenger seat and compare the decisions made by the driver to those you would make.

Chris

[lyndon]

Not sure whether you're currently doing an advanced course or have completed one so apologies if I'm teaching you to suck eggs, but one way to re-evaluate/recalibrate your own judgements is to drive from the passenger seat and compare the decisions made by the driver to those you would make.
Chris

I've taken the IAM test, but I never did learn to suck eggs (neither did my Grandmother, as far as I know, but that is another story). But I'm still at the start of my journey, and trying to stay ahead of senility at the same time.

I like the idea of 'recalibrating your judgement'. Yes, that sums up what I'm trying to do very well. I did start the process with trying to estimate the distance to the limit point when cornering, and hence determine my absolute maximum safe entry speed. Then I realised that my 'without drama stopping distance' is very well calibrated, and gets checked every single time I come to rest. But there are a few areas that I feel would benefit from a more analytical approach. Overtaking is one (Done that one. The maths are bit more complex though), and my T junction question is the other.

[GJD]

But there are a few areas that I feel would benefit from a more analytical approach. Overtaking is one (Done that one. The maths are bit more complex though), and my T junction question is the other.

If you want to approach it analytically, would it help to find a quiet road and time how long it actually takes you to exit the side road and get up to 60? And/or measure how far you actually travel getting up to 60? In your original post it seemed like you were asking for opinions on the assumptions you were making, but perhaps you could gather some actual data to use as a starting point.

[playtent]

I imagine like me many people throttle back a bit if someone turns out in front of them from a side road just in case. So that vehicle may have slowed to 50mph by the time you reach 60mph. Then again some people put their foot down to prove their point that you pulled out in front of them and now they are right up your chuff flashing their lights!
So because humans are the unknown factor in this equation:

U=myo2myw

U= unkown
m=maybe
y=you'll be/ you
o=ok
w=won't
2=or

Sorted that one, whats next?

[playtent]

0-60 in 8 seconds?, may well be do-able but on a dry perfect surface, and no way whilst turning as well, add to that white lines (no grip) man-hole covers (no grip) patched tarmac (no grip) etc etc...add at least another 10 secs

Depends what he's driving I would think?

4 Wheel drive...at best a snapped drive shaft will result in you doing sweet f.a untill the oncoming car hits you.

Why would giving it the beans in that way snap a drive shaft?

[IVORTHE DRIVER]

[quote="playtent"

Why would giving it the beans in that way snap a drive shaft?[/quote]


I think if you floor it whilst turning and one wheel hits a manhole cover as the other 3 are gripping like crazy something is going to break.

Nice to see he's not a "boy racer" though so guess it might not be a wind up after all, apologies for any offence OP.

Still have to ask a question though.......why pull out, whats wrong with holding a bit longer?

Also agree with whoever said "most of us would lift off anyway", in some cases probably even before OP had thoght of pulling out

Wonder if thats "intuition"?

Ivor

[GJD]

Still have to ask a question though.......why pull out, whats wrong with holding a bit longer?

The OP said in one of his later posts that he thinks his judgement is too conservative and he's being encouraged not to be over-cautious at T junctions. I don't think he's looking to eliminate cases of pulling out when he ought to have waited. I think it's the other way around.

Also agree with whoever said "most of us would lift off anyway", in some cases probably even before OP had thoght of pulling out

Really? If I suspect someone's going to pull out in front of me, I start planning my overtake. How's lifting off gonna help with that?