Article: Five Tips for Safer Driving

"Floyd Rogers" > writes:
>"Scott en Aztlán" > wrote
>> "Floyd Rogers" > said
>>>"Bernd Felsche" > wrote

>< ... getting too long..

Eh? You guys WANTED to complicate the issue.

And the crux of the matter is that complicating the issue and taking
"everything" into account, pretty much produces the same result.

>I knew that the WA DOT had a lot of data on traffic volumes
>and flow; was poking around last night. This morning I poked
>some more and found a FABULOUS document:
>http://www.wsdot.wa.gov/rail/plans/DTA/DTAch_two.cfm
>Table 2.2 relates service level to speed and maximum flow.
>(I don't see 1800 anywhere ;->) Interestingly, maximum
>flow goes *UP* as speed/congestion goes *DOWN* -
>presumably due to closer spacing.

Critically so. 2400 VPH @ 53 mph ?? 1.5 second intervals?

What did I suggest you do about people who rate traffic volumes that
high? And it is a rating! (v/c ratio = 1.0)

The speeds reduce because of "side friction"; the perception of
needing to drive "more carefully" when there are other vehicles
around. The free-flowing volumes tend to peak at about 1800 VPH.
It is pretty much when the highest aggregate speeds are achieved,
calculated by multiplying the number of vehicles by their speed.

Higher speeds are observed when there is less _demand_ for the road
which of course doesn't _require_ as many vehicles per hour.
i.e. the freeway is not "full".
--
/"\ Bernd Felsche - Innovative Reckoning, Perth, Western Australia
\ / ASCII ribbon campaign | "Laws do not persuade just because
X against HTML mail | they threaten."
/ \ and postings | Lucius Annaeus Seneca, c. 4BC - 65AD.

"Floyd Rogers" > writes:
>"Bernd Felsche" > wrote

>Well, we finally got a straight answer: "it all depends". ;->
>All these figures for capacity are empirically measured and are
>"close" to a theoretical figure, and that Scott's equation is as
>good as any used by guys with PhD's, and is as good as your 1800
>v/h figure.

Scott does a lot more work for a worse result. :-)

>One nit I have is your use of "interval between vehicles". To me,
>the use of the adjective "between" strongly implies the time
>between the rear of the front car, and the front of the following.

"It depends". Do you check the interval or the spacing to the car in
front of yours? And how accurately do (not _could_) you judge that
interval? Are you in a position to determine when the back of the
car in front passes a fixed point on the road(-side) and when the
front of your passes the same?

>IMO, no other concept can be implied by that use. Using
>"frequency" would be a much better choice to imply front-end to
>front-end timing. I believe that Scott would agree.

The difference "comes out in the wash".

As I said before; the tolerance in drivers judging the spacing is
greater than the length of a passenger car at (free-flowing) freeway
speeds.
--
/"\ Bernd Felsche - Innovative Reckoning, Perth, Western Australia
\ / ASCII ribbon campaign | "Laws do not persuade just because
X against HTML mail | they threaten."
/ \ and postings | Lucius Annaeus Seneca, c. 4BC - 65AD.

"Bernd Felsche" > wrote
> "Floyd Rogers" > writes:
>>"Bernd Felsche" > wrote
>
>>Well, we finally got a straight answer: "it all depends". ;->
>>All these figures for capacity are empirically measured and are
>>"close" to a theoretical figure, and that Scott's equation is as
>>good as any used by guys with PhD's, and is as good as your 1800
>>v/h figure.
>
> Scott does a lot more work for a worse result. :-)
>
>>One nit I have is your use of "interval between vehicles". To me,
>>the use of the adjective "between" strongly implies the time
>>between the rear of the front car, and the front of the following.
>
> "It depends". Do you check the interval or the spacing to the car in
> front of yours? And how accurately do (not _could_) you judge that
> interval? Are you in a position to determine when the back of the
> car in front passes a fixed point on the road(-side) and when the
> front of your passes the same?
>
>>IMO, no other concept can be implied by that use. Using
>>"frequency" would be a much better choice to imply front-end to
>>front-end timing. I believe that Scott would agree.
>
> The difference "comes out in the wash".
>
> As I said before; the tolerance in drivers judging the spacing is
> greater than the length of a passenger car at (free-flowing) freeway
> speeds.

No, IMO, the "tolerance" you mention is inherent in the empirical
data - the scatter graphs - that the researchers used to generate
their equation. Because that data includes actual spacing - that is
apparently greater than a 2 "perfect" sec front-to-front spacing -
the results gave capacity figures (3500 vph for a 2-lane) that is
less than the "perfect" 1800 vph.

FloydR

"Floyd Rogers" > writes:
>"Bernd Felsche" > wrote
>> "Floyd Rogers" > writes:
>>>"Bernd Felsche" > wrote

>> "It depends". Do you check the interval or the spacing to the car in
>> front of yours? And how accurately do (not _could_) you judge that
>> interval? Are you in a position to determine when the back of the
>> car in front passes a fixed point on the road(-side) and when the
>> front of your passes the same?

You didn't answer any of these simple questions.

>>>IMO, no other concept can be implied by that use. Using
>>>"frequency" would be a much better choice to imply front-end to
>>>front-end timing. I believe that Scott would agree.

>> The difference "comes out in the wash".

>> As I said before; the tolerance in drivers judging the spacing is
>> greater than the length of a passenger car at (free-flowing)
>> freeway speeds.

>No, IMO, the "tolerance" you mention is inherent in the empirical
>data - the scatter graphs - that the researchers used to generate
>their equation. Because that data includes actual spacing - that is
>apparently greater than a 2 "perfect" sec front-to-front spacing -
>the results gave capacity figures (3500 vph for a 2-lane) that is
>less than the "perfect" 1800 vph.

Oh yeah; 1750 VPH... close enough. The German studies seem to
indicate that more than 2000 VPH tends to lead to a "collapse" in
throughput, with vehicle speeds and VPH reducing; perhaps even
catastrophically to congestion (traffic jam).

One can hypothecise that is to be expected as close spacing of
vehicles will result in the frequent use of brakes and the ripple of
braking becomes more severe the greater the length of the
closely-spaced train. A longer "cushion" ahead of the vehicle allows
the perceptive drivers to usually adjust speeds without touching the
brake.

There is a tolerance in every measurement, be that judged or
measured by instruments.

Measurements by instrument; measuring traffic speeds and counting
the number of vehicles; are generally better than +/-10% but
probably greater than +-3%; especially speeds. Instrument
measurements tend to be scattered around the mean which they
indicate, so unless the instruments haven't been calibrated
correctly (or are being use incorrectly), the +/- errors tend
to average out when a large number of measurements are taken.
--
/"\ Bernd Felsche - Innovative Reckoning, Perth, Western Australia
\ / ASCII ribbon campaign | "Laws do not persuade just because
X against HTML mail | they threaten."
/ \ and postings | Lucius Annaeus Seneca, c. 4BC - 65AD.

Scott en Aztlán > writes:
>Bernd Felsche > said in aus.cars:

>>>I knew that the WA DOT had a lot of data on traffic volumes
>>>and flow; was poking around last night. This morning I poked
>>>some more and found a FABULOUS document:
>>>http://www.wsdot.wa.gov/rail/plans/DTA/DTAch_two.cfm
>>>Table 2.2 relates service level to speed and maximum flow.
>>>(I don't see 1800 anywhere ;->) Interestingly, maximum
>>>flow goes *UP* as speed/congestion goes *DOWN* -
>>>presumably due to closer spacing.

>>Critically so. 2400 VPH @ 53 mph ?? 1.5 second intervals?

>How ironic that you are now asking the EXACT same questions that I
>asked earlier in this thread!

Yeah... nice snip to attempt to troll.

You snipped:
>> What did I suggest you do about people who rate traffic volumes that
>> high? And it is a rating! (v/c ratio = 1.0)

>As I recall, you called me "stupid" for asking those kinds of
>questions... What does that make you?

I didn't call you "stupid".

Read what I wrote; not what you read.

[PLONK!]
--
/"\ Bernd Felsche - Innovative Reckoning, Perth, Western Australia
\ / ASCII ribbon campaign | "Laws do not persuade just because
X against HTML mail | they threaten."
/ \ and postings | Lucius Annaeus Seneca, c. 4BC - 65AD.

"Bernd Felsche" > wrote
> "Floyd Rogers" > writes:
>>"Bernd Felsche" > wrote
>>> "Floyd Rogers" > writes:
>>>>"Bernd Felsche" > wrote
>
>>> "It depends". Do you check the interval or the spacing to the car in
>>> front of yours? And how accurately do (not _could_) you judge that
>>> interval? Are you in a position to determine when the back of the
>>> car in front passes a fixed point on the road(-side) and when the
>>> front of your passes the same?
>
> You didn't answer any of these simple questions.

I thought that those were rhetorical questions, not that you actually
want an answer. The answer is "yes, I am in a position to tell if
a car passes a point on the road, and I can discriminate between
the front and rear". Roads in much of the US have "turtles" or
reflectors in the lane markings that allow this. And at the usual
30mph on surface streets, 15-20 feet is a very significant part of
the 44ft/s velocity.

FloydR

In article om>,
gpsman > wrote:
>Matthew Russotto wrote:
>> In article >,
>> Bernd Felsche > wrote:
>> >
>> >A freeway is "full" when all lanes are at 1800 vehicles/hour.
>> >
>> >That allows for a 2-second gap between vehicles. If you try to
>> >squeeze more on, then the time interval between vehicles and safety
>> >reduces.
>>
>> The 2-second rule is arbitrary. I've seen and driven on "freeways"
>> traveling at speed at sub-second intervals.
>
>Your anecdotal evidence suggests only that it's violated arbitrarily.
>
>The 2 second rule is actually 3 seconds (US) under optimal conditions,

Cite?

>and is not arbitrary. It's based on driver reaction time and vehicle
>performance. Do you think someone just one day picked a number from
>thin air and nobody... ever... asked why?

Actually, yes. Or, while they may have asked, they didn't get a satisfactory
answer.
--
There's no such thing as a free lunch, but certain accounting practices can
result in a fully-depreciated one.

In article om>,
gpsman > wrote:
>Matthew Russotto wrote:
>> In article >,
>> Bernd Felsche > wrote:
>> >
>> >A freeway is "full" when all lanes are at 1800 vehicles/hour.
>> >
>> >That allows for a 2-second gap between vehicles. If you try to
>> >squeeze more on, then the time interval between vehicles and safety
>> >reduces.
>>
>> The 2-second rule is arbitrary. I've seen and driven on "freeways"
>> traveling at speed at sub-second intervals.
>
>Your anecdotal evidence suggests only that it's violated arbitrarily.
>
>The 2 second rule is actually 3 seconds (US) under optimal conditions,
>and is not arbitrary. It's based on driver reaction time and vehicle
>performance. Do you think someone just one day picked a number from
>thin air and nobody... ever... asked why?

Yes. Well, yes to the arbitrary part. People do ask why.
There is no following distance about which you can say
"less is dangerous, more is safe." There are so-called
"experts" who will testify in court that 2.000 seconds,
or 3.000 seconds, is that distance, but they're no more
worth listening to than the so-called accident reconstruction
experts who testify with equal certainly that the car
was moving precisely 56.194 miles per hour based on some
calculation that has barely one significant digit of
accuracy.

There is no bright line. By general agreement among drivers,
the only people whose opinion matters, one to one and a half
seconds is a reasonable following distance. Maybe one and a
half is better than one if you can manage. Half a second is
obnoxious. Two seconds is too long in heavy traffic.
Suggesting that three seconds should be the minimum reveals
a lack of understanding of driving in places where people
outnumber cows.

Highway design standards assume that people will follow
more closely than two seconds. That's because the standard
assumes human drivers, not programmable machines. A typical
average following distance in heavy traffic is 1.5 seconds.
That's an average, not a minimum. It's going to happen, no
matter how much you huff and puff and blow "three seconds!"

Of course, it's possible to cause a crash by slamming on
your brakes, just like you can cause a crash by jerking
the steering wheel when somebody is beside you, or accelerating
forward when cross traffic is approaching. All of these are
illegal when done unsafely.

--
John Carr )

"Dave en Aztlán" > writes:

>Scott en Aztlán > said in rec.autos.driving:

>>And now you've PLONKed me. My, my, my... What a sore loser you are.

>Indeed.

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/"\ Bernd Felsche - Innovative Reckoning, Perth, Western Australia
\ / ASCII ribbon campaign | "Laws do not persuade just because
X against HTML mail | they threaten."
/ \ and postings | Lucius Annaeus Seneca, c. 4BC - 65AD.

John F. Carr wrote:
> In article om>,
> gpsman > wrote:
> >Matthew Russotto wrote:
> >> In article >,
> >> Bernd Felsche > wrote:
> >> >
> >> >A freeway is "full" when all lanes are at 1800 vehicles/hour.
> >> >
> >> >That allows for a 2-second gap between vehicles. If you try to
> >> >squeeze more on, then the time interval between vehicles and safety
> >> >reduces.
> >>
> >> The 2-second rule is arbitrary. I've seen and driven on "freeways"
> >> traveling at speed at sub-second intervals.
> >
> >Your anecdotal evidence suggests only that it's violated arbitrarily.
> >
> >The 2 second rule is actually 3 seconds (US) under optimal conditions,
> >and is not arbitrary. It's based on driver reaction time and vehicle
> >performance. Do you think someone just one day picked a number from
> >thin air and nobody... ever... asked why?
>
> Yes. Well, yes to the arbitrary part. People do ask why.
> There is no following distance about which you can say
> "less is dangerous, more is safe."

Uh, no to the arbitrary part. The "average" reaction time of drivers
under optimal conditions has been considered to be 1.5 seconds for
decades, if not half a century. Doubling that time/distance as a
cushion has long been accepted as reasonable and inexcessive.

> Suggesting that three seconds should be the minimum reveals
> a lack of understanding of driving in places where people
> outnumber cows.

Or an understanding of reaction times and vehicle performance.

> Highway design standards assume that people will follow
> more closely than two seconds.

Uhhh... cite please.

> A typical
> average following distance in heavy traffic is 1.5 seconds.

Possibly, maybe probably..

> That's an average, not a minimum. It's going to happen, no
> matter how much you huff and puff and blow "three seconds!"

That doesn't happen to me in a sustainable manner. My following
distance is a minimum of 2 seconds or I make it 2 seconds.

> Of course, it's possible to cause a crash by slamming on
> your brakes

> (this is)
> illegal when done unsafely.

Please define "unsafe" braking.

Please cite an instance where slamming on your brakes to avoid an
obstacle or collision is illegal when performed "unsafely".

Does that mean if you're 1.5 seconds off my ass and a crash occurs to
my front and I slam on my brakes and you collide with me I am "at
fault"... for slamming on my brakes "unsafely".?

Thank you for your kind attention to this matter.
-----

- gpsman