Prevailing Speed vs Posted Speed Limit

Andrew Tompkins wrote: <brevity snip/groups adjusted>
> Another factor that goes into the
> equation is the fact that we do need to move ourselves and our stuff
> around in a reasonable amount of time.

How does "reasonable amount of time" figure into the equation? AFAICT
everybody's travel time is "reasonable", all things considered. Most
people, I think, would disagree.

I think the measure of "reasonable" as in "the fewest number of
seconds" is at fault for a large number of problems with traffic and
the cause of many crashes, if not most.

One man's "reasonable" is another man's "too fast" or too slow".
-----

- gpsman

On 29 Aug 2006 14:51:15 -0700, "Old Wolf" >
wrote:

>Speeders & Drunk Drivers are MURDERERS wrote:
>> On 28 Aug 2006 11:37:41 -0700, "morticide" > wrote:
>> >
>> >It's nasty either way...go with the flow and risk being in a
>> >multiple-car pileup, or go slow and risk collision from behind.
>> >Neither method is 100% safe.
>>
>> I never get read-ended. NEVER. No question that slower is safer.
>>
>> The way to get rear-ended is to make panic stops. Driving slow
>> doesn't do it.
>
>I drive fast and I've never been rear-ended. NEVER. No question.
>Now what?

Yeah but how many cars have YOU rear-ended? Thousands, no doubt.

On 29 Aug 2006 09:52:15 -0700, "Larry Bud" >
wrote:

>
>Speeders & Drunk Drivers are MURDERERS wrote:
>> On 28 Aug 2006 11:40:12 -0700, "Larry Bud" >
>> wrote:
>>
>> >
>> >Speeders & Drunk Drivers are MURDERERS wrote:
>> >> On Mon, 28 Aug 2006 11:58:36 -0500, Bob Simon >
>> >> wrote:
>> >>
>> >> >I am trying to convince my wife that if safety is your highest
>> >> >concern, then it is better to drive the prevailing speed than the
>> >> >speed limit. If this is in fact true, can anyone point me to an
>> >> >authoritative source?
>> >>
>> >> It's called common sense.
>> >
>> >That's true, and it's common sense that if everybody is driving 70,
>> >that driving 50 is a hell of a lot more dangerous than keeping pace
>> >with everybody else at 70.
>> >
>>
>> Nothing common-sensical about that at all. Just the opposite. The
>> fact that so many drivers are reckless speeders gives an intelligent
>> person all the more incentive to slow down so they can avoid all the
>> crashes caused by the speeders.
>
>It's harder to avoid crashes when your speed differential is 20 mph
>rather than if it were 0 mph of the action. But I don't expect you to
>understand.

Nope - slower is safer.

>From http://casr.adelaide.edu.au/speed/vol-1.html#sec1.3:

2.4 Summary
Evidence from correlational studies suggests there is a positive
association between speed and crash involvement. This type of study is
unable to provide full details of the relationship, however.

Three studies conducted in the United States more than 25 years ago
attempted to enumerate the relationship between speed and crash
involvement using data from individual crashes. These studies concluded
that crash involvement was a U-shaped function of vehicle travelling
speed. The studies were subject to methodological problems, with the
consequence that the meaning of the results is not certain. In
particular, it is debatable as to whether the elevated involvement
rates found at low speeds were due to bias, to vehicles undertaking
slow manoeuvres, or to drivers genuinely electing to travel slowly.

Studies which have linked drivers' speeds and accident histories have,
on the whole, not supported a U-shaped relationship between speed and
crash involvement. In particular, a recent Australian study found that
the slowest drivers had the least experience of crashes, while the
fastest drivers had the greatest experience of crashes.

Progress in determining the nature of the relationship in question
would appear to require further use of the most direct study design,
bearing in mind and attempting to overcome past deficiencies. Estimates
of pre-crash speed are a potential weakness of the approach, but this
could be improved using contemporary accident reconstruction methods
and computer programs (in the absence of a representative and valid
cohort study using 'black box' speed recorders). In addition, care
should be taken to ensure that all vehicles contributing to the data
set were actually travelling at a free speed and that controls are
truly comparable.

When you go into a product review meeeting, do you wheel in a cart with
a huge pile of books, two third of which are unrelated to your design,
claim they prove your point, and then defie anyone who challenges your
assertion to read them all and provide a detail scholarly review of why
they don't support you? This is what you did to me when I asked for
proof that the 85th percentile rule was the safest way to set speed
limits. Most of the list you posted, if they mentioned the 85th
percentile rule at all (and many did not), only did what you are doing,
quote the 85th percentile rule as the best method for setting speed
limits. The one study that some seem to think justifies this rule is
very old, and has its own problems (see
http://casr.adelaide.edu.au/speed/vol-1.html#sec1.3 )

To be very clear about what I think -

1) I do agree with the idea that vehicles going significantly below the
average speed of traffic can lead to an increase in accidents.
2) Likewise vehicles going significantly faster than the average speed
of traffic can also lead to an increase in accidents
3) Agressive, impatient, and/or angry drivers are a greater danger than
calm people going slower or faster than average. Slow drivers that stay
out of the way cause less problems than agressively slow drivers who
refuse to get out of the way. Likewise agressive drivers who try to
bully their way through traffic by tailgaiting, flashing lights,
weaving from lane to lane, etc, are far more dangerous than faster
drivers who can show a little courtesy and patience when trying to work
past slower traffic.
4) People who are responsible for accidents are often not involved in
the accidents they trigger
5) When I say the 85th percentile rule is arbitrary, I am using the
definition of arbitrary that says "based on or determined by individual
preference or convenience rather than by necessity or the intrinsic
nature of something ." I believe the rule is arbitrary becasue there is
insufficient proof that it is "the best" rule for setting speed limits.

6) I don't know the best way to set speed limits to maximize the
benefit to society. Wasting time is bad, accidents are bad, wasting gas
is bad, etc. However, I think a lot more should figure into setting
speed limits than the notion that the 85th percentile rule is the best
way to do it, particulalry since I think this rule is based on shaky
assumptions. To me it is a crutch used to avoid considering all the
factors.
7) If speed limits aren't obeyed, it doesn't matter what rule is used
to set them.

Ed

In article .com>, Ed White wrote:
>
> When you go into a product review meeeting, do you wheel in a cart with
> a huge pile of books, two third of which are unrelated to your design,
> claim they prove your point

1) this is usenet, not a design meeting.
2) I didn't sift through the list I just read it into the post. It's not
my fault he hasn't been paying attention in the group.
3) I don't spend time teaching fellow engineers basics they should have
learned for their BS degrees.

> and then defie anyone who challenges your
> assertion to read them all and provide a detail scholarly review of why
> they don't support you? This is what you did to me when I asked for
> proof that the 85th percentile rule was the safest way to set speed
> limits. Most of the list you posted, if they mentioned the 85th
> percentile rule at all (and many did not), only did what you are doing,
> quote the 85th percentile rule as the best method for setting speed
> limits.

Because I'm tired of going over the same old stuff time and time and time
and time again each time someone comes in and decides to poke. I wish I
had saved the posts of a particular former regular who knew every
detailed study and quoted and cited them. Maybe I'll find his posts out
of the google archive and list them. I don't feel it's my responsibility
to review in detail the last ten years of this newsgroup for everyone who
wanders in here individually.

> 5) When I say the 85th percentile rule is arbitrary, I am using the
> definition of arbitrary that says "based on or determined by individual
> preference or convenience rather than by necessity or the intrinsic
> nature of something ." I believe the rule is arbitrary becasue there is
> insufficient proof that it is "the best" rule for setting speed limits.

There is never proof that something is best, only the best we know. The
simple way to disprove that is come up with something better that is
better supported. But nothing is ever offered as better supported,
nothing but nit picking and sniping at flaws and bringing up special cases.

All the sniping in the world, all the pointing out of flaws will change
it as the best known method. What will change that is providing something
that has more support backing it with fewer flaws. So set your sights on
that instead of the same old tired discussion of the 85th percentile vs a
number from some elected offical's ass or some civil engineering table
from 1932. As old as the 85th percentile method is, it's newer than those
two methods.

> the notion that the 85th percentile rule is the best
> way to do it, particulalry since I think this rule is based on shaky
> assumptions. To me it is a crutch used to avoid considering all the
> factors.

Where's a better method with more solid footing? It's real easy for you
to knock this down, just produce the better method and present it.

> 7) If speed limits aren't obeyed, it doesn't matter what rule is used
> to set them.

If speed limits don't make sense to the vast majority, they won't be obeyed.

Speeders & Drunk Drivers are MURDERERS wrote:
> On 29 Aug 2006 09:52:15 -0700, "Larry Bud" >
> wrote:
>
> >It's harder to avoid crashes when your speed differential is 20 mph
> >rather than if it were 0 mph of the action. But I don't expect you to
> >understand.
>
> Nope - slower is safer.


Not when you're out doing that 20mph under the prevailing traffic. In
that situation, you're a rolling roadblock and it will only be a matter
of time before you cause an accident.

If you can't drive with the traffic, then don't. Take the back roads
instead.

> >> Nothing common-sensical about that at all. Just the opposite. The
> >> fact that so many drivers are reckless speeders gives an intelligent
> >> person all the more incentive to slow down so they can avoid all the
> >> crashes caused by the speeders.
> >
> >It's harder to avoid crashes when your speed differential is 20 mph
> >rather than if it were 0 mph of the action. But I don't expect you to
> >understand.
>
> Nope - slower is safer.

So why are you even doing 50mph?

In article .com>,
"Ed White" > wrote:

>
> 6) I don't know the best way to set speed limits to maximize the
> benefit to society. Wasting time is bad, accidents are bad, wasting gas
> is bad, etc. However, I think a lot more should figure into setting
> speed limits than the notion that the 85th percentile rule is the best
> way to do it, particulalry since I think this rule is based on shaky
> assumptions. To me it is a crutch used to avoid considering all the
> factors.

The only legitimate (non-monetary) reason for a speed limit is to advise
motorists of a condition ahead that would be hazardous to a car
traveling above a certain speed - such as in a curve. On a
well-designed, modern and straight interstate, there is no need for any
speed limits - under these conditions, all speed limit numbers are
arbitrary.


> 7) If speed limits aren't obeyed, it doesn't matter what rule is used
> to set them.

If a speed which is not exceeded by the majority of motorists is posted,
than that speed limit is being obeyed.

In article >, wrote:

>> 6) I don't know the best way to set speed limits to maximize the
>> benefit to society. Wasting time is bad, accidents are bad, wasting gas
>> is bad, etc. However, I think a lot more should figure into setting
>> speed limits than the notion that the 85th percentile rule is the best
>> way to do it, particulalry since I think this rule is based on shaky
>> assumptions. To me it is a crutch used to avoid considering all the
>> factors.
>
> The only legitimate (non-monetary) reason for a speed limit is to advise
> motorists of a condition ahead that would be hazardous to a car
> traveling above a certain speed - such as in a curve. On a
> well-designed, modern and straight interstate, there is no need for any
> speed limits - under these conditions, all speed limit numbers are
> arbitrary.

Furthermore, if speed limits made sense by meeting what most people find
to be reasonable, when a driver saw one that was lower than normal, and
every lower than normal speed limit he ever saw was there because of
some hazard up ahead that required it, he would obey it.