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Old 08-05-18, 07:26 AM   #1
Seeker
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Default How long to pass a car?

In view of my recent encounter with a camera van and the fact that the police prefer to refer to them as Safety Cameras instead of Speed Cameras, I thought I'd crunch a few numbers to see how safe keeping to the speed limit actually is.

Sadly, I was too lazy to resolve a formula myself, so I turned to google and the help of Dr. Math.

http://mathforum.org/library/drmath/view/56330.html

Two examples both assuming that:
1) the car being overtaken is doing a constant 55 mph
2) it takes about 12 car lengths to get past the car and the car is 15 feet long, giving a distance to be covered of 180 feet (d)
3) the passing vehicle is also doing a constant speed (v2)


The formula is required to calculate D, distance needed to pass

D = d x v2/(v2-v1)

Example 1

The passing vehicle (v2) is doing 60 mph
the overtaken vehicle (v1) is doing 55mph


D = 180 x 60(60 - 55) = 2160 feet (0.4 mile)

from this we work out how long it would take to pass since 60 mph = 88 ft/sec

2160/88 = 24.5 secs.

Example 2 (my case)

The passing vehicle v2 is doing 70 mph
the overtaken vehicle is doing 55 mph

D = 180 x 70 (70-55) = 840 feet (0.16 mile)

and the time? 70 mph = 102.6 ft/sec

840/102.6 = 8.2 secs

So:
at 60 mph it requires 2160 ft and 24.5 secs
at 70 mph it requires 840 ft and 8.2 secs


I could imagine going to court with this, watching the court's eyes glaze over and then told to pay the fine anyway
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Old 08-05-18, 08:05 AM   #2
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Default Re: How long to pass a car?

There's a simpler answer, if you can't complete the overtake safely while remaining within the speed limit just sit behind.
And this would be what the court would tell you in response to your arithmetic.
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Old 08-05-18, 09:49 AM   #3
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Default Re: How long to pass a car?

If a vehicle was doing 55 in a 60 limit I would probably not pass it unless it was a diesel blowing smoke at me (even then depending upon how straight or twisty the road was I may just drop back). If it was taking the p!55 doing 40, definitely would.
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Old 08-05-18, 09:49 AM   #4
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Default Re: How long to pass a car?

Yet during my driving lessons (a long time ago) and during various bike training courses (including a bike safe course run by a bike cop) I've been told it is acceptable to briefly go over the posted limit to complete the overtake, unless you're doing that to squeeze in an overtake that is not really safe. The above being said, my own interpretation is that it is only safe on NSL roads.
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Old 08-05-18, 03:13 PM   #5
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Default Re: How long to pass a car?

The thing is, the scumbags who operate the camera vans know exactly where the decent overtaking spots* are, so will prioritise them.
It's the best way to get most value out of the motorist for their nonce mates who run the outsourcing companies who run the speed awareness courses.

Logic and safety are not factors in this.



*I'm sorry I must have meant accident black spots. Like the one near halifax where loads of people crash when it snows and they confuse "excessive speed for the conditions" with "speeding". Obviously what you need to avoid a load of 10mph fender benders is to reduce the limit to 40 and put a gimp in a van there...
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Old 08-05-18, 03:33 PM   #6
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Default Re: How long to pass a car?

A lot of the camera van haunts have a grey electrical box at side of road and some cables buried in the road, these continuously monitor traffic speed and log the data. The Police (or camera partnership) can access this data and if there have been enough vehicles speeding they will put a van there for a day. I am sure some of the flashing speed signs we see in 30 and 40 limits also collect data for speed of traffic passing through them.

It may seem random where the speed vans park up, but spots are carefully chosen to catch greatest number of speeders.

Speeding up to try to prevent someone overtaking is on of the more nasty things drivers do, they can be doing 40 but when they see you are trying to pass (or approaching a straighter stretch of road) they will speed up, and probably slow down again if you drop back behind them.
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Old 08-05-18, 04:19 PM   #7
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Default Re: How long to pass a car?

IAM say never break the speed limit, RoSPA say get the overtake done as quickly as possible. While speed cameras have no powers of reasoning, a copper would be hard pressed to want to book you for speeding up an overtake, provided of course you're not just riding like a tool anyway.

The flip side of claiming speed limits are universally applicable is that some people think they must go at the speed limit, even when this is too fast for the road or traffic (including non-car users).
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Old 08-05-18, 04:51 PM   #8
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Default Re: How long to pass a car?

I was told by a bike cop a few years ago that they didn't worry in my neck of the woods in NSL until you went over 70 and were being dangerous. Otherwise they only worried about areas with signed restrictions.
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Old 08-05-18, 05:31 PM   #9
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Default Re: How long to pass a car?

problem with scamera vans and bikes is that they need to catch your registration number either using recognition or video cameras.

good news is that they cant get you from the front and only the rear.

i think they only take "snap shots" at certain points to generate the speed record otherwise the cameras would be recording every single vehicle in a line of traffic all at once, thats a lot of data.

so they cant record your speed until they can recognise your number plate, so bikes can only be recorded with "rear" facing cameras/recognition but these also have to be tied in with the "front" ones to prove that you were speeding before hand.

i have passed a few scameravans overtaking and to this day nothing has ever come of it.
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Old 09-05-18, 06:09 AM   #10
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Default Re: How long to pass a car?

Reality of it is you were just unlucky.
Most of the time your get away with it.
Just try not to let it spoil the enjoyment.
Book a few track day's end enjoy the summer
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