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Green waves of traffic lights on several intersecting streets
are either problematic, PITA or impossible to combine.
Unless you have an approximately square grid pattern of
streets and avenues, not completely unheard of in some
parts of the world.
Let's imagine a block length of 200 yards.
Now
for a traffic light cycle of 20 seconds (7green-3yellow-
10red) you get a speed limit of 200yds/20s = 20mph.
Not exactly zippy but acceptable for city if you never need
to sit in the reds. And this applies for every street and
avenue in both directions when you phase the lights right.
Right turns are less problematic but all left turns will have
to be executed as Michigan turns.
Now let's fire up the bakery oven for all the issues that
would arise in practice.
Eternal Green Wave animation
http://i.imgur.com/BcwCVFB.gifv
(epilepsy inducing content) [AK-74, Sep 20 2015, last modified Sep 23 2015]
Traffic Engineering
https://en.wikipedi...ng_(transportation)
[MechE, Sep 21 2015]
Excellent photos of Lombard. If you've never been, it's worth it just for fun.
https://foursquare....647e4b07cca83f592ae
[blissmiss, Sep 22 2015]
bidding_20for_20greens
[hippo, Sep 23 2015]
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Annotation:
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Also, welcome to the halfbakery. |
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e's sayin' that there should be a smartphone app, relying on gps and tied in through the web to traffic control computers, that you can access as you drive along, to give the optimum speed at any given point, by navigating through a series of ads and a couple captchas. |
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Oh dear, I'm hopeless in expressing myself. After
rereading the opening I must agree it's indeed badly
written.
sigh ... gets up and tries again - |
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I claim that it's possible to time stoplights in an ideal grid
plan city so that everyone has a green wave, all the time.
Conditions are:
1) speed limit must be [stoplight distance/light cycle
time]
2) All avenues' lights run in the same cycle with each
other
3) Streets' lights must obviously do the same too
4) Left turns are done by a 'michigan turn'
5) There must be some problems. Lots of them actually.
And I've sharpened my pen to fight them. |
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Yes, I'll maybe have to illustrate this. |
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You need to account for acceleration time, to reach the
ideal speed. Anyone turning a corner will have a change-
of-velocity factor (can't go around the corner at full
speed). |
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On the other hand, it is possible to design/build the traffic
system with no lights at all, "simply" by having overpasses at
every intersection. That's another way to keep all the
traffic moving. |
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I posted this idea once... I think. |
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The responses indicated that this is what is currently done if everyone is driving the speed limit, but that it only works in one direction so the lights are supposed to favor the heaviest flow of commuters. |
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If you really want the lights to all stay green for you just rig up a strobe light to the roof of your car and all of the lights will change for you as though you are an emergency vehicle... right up until you get pulled over by the cops for having a strobe light on your roof. |
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//pulled over by the cops for having a strobe light on
your roof.// |
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What's the spectral response of the sensors? You
might be able to do this with a UV strobe - invisible
to the policemen. |
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I'm not sure. I figured this out by accident with a broken high-beam switch I was flicking in frustration at a red light when I was the only driver around.
If you want the specs just search for "Preemptive signal changers". The latest blogs on the subject show that individual pulses differentiate specific emergency vehicles ever since people started rigging up their own gear, so it is becoming more difficult to get away with. |
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Posted an illustration (how do you use animated GIFs as
desktop background by the way?) and noticed a mistake
in my calculations: speed limit is TWO block lengths
divided by light cycle time. That's convenient. 200 yards
block would have been a hefty one. For manors or a
stadium perhaps. |
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//can't go around the corner at full speed//
Don't need to. You are ahead of the green wave after a
turn. Just slow down to get a flying start at the next
lights. |
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//During rush hour though, all roads are filled so the
whole system is screwed//
Most definitely. |
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That's a fabulous animation. Nice first posting.+ |
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If no one ever gets hung up at the light, are they necessary? |
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hehehehe... That's an awesome video. Thank you. |
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// One solution is to treat all roads like a square grid// |
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Which isn't possible because of situations like DC's
avenues that can cross a north-south road and then an
east-west road in immediate succession, where those
two roads cross each other at the next light. |
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To say nothing of Boston where it's possible to turn left,
then right and easily end up on the road you just left,
heading in the opposite direction. |
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Put simply, there is a whole branch of civil engineering
that specializes in this area, and what we've got is the
best they can produce. |
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Relevant things to consider with regards to the original
concept: |
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What happens when someone pulls out or into parking in
the middle of the street, and how do you account for the
slowdown? |
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What happens if a pedestrian is slightly slow to cross the
street at an intersection, and the resulting slowdown? |
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Ball bouncing across the street? Squirrel? |
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Driver driving below speed because they are looking for a
parking space or an address? |
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Emergency vehicle slows up traffic in one direction? |
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Anything that causes a partial holdup, especially one that
ends up being longer than the block in question is going to
result in an expanding pulse of slowness that will
eventually engulf the entire city. |
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Welcome Wagon says howdy [AK-74] . I love the idea for
driving home at night from work, when there are no cars and I
sit at red lights forever, for no apparent reason, wasting my
time, and Gods. Well...at least mine. |
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//Put simply, there is a whole branch of civil
engineering that specializes in this area, and what
we've got is the best they can produce.// |
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Right. But traffic management does evolve over
time. And even the c-students get picked up by
some locale. |
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//You might want to read the remainder of that anno.// |
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Look at New Hampshire Avenue between S and T NW in
Washington DC. There is no possible way to drive slow
enough to treat those sections as a uniform grid, the
acceleration and deceleration times alone would kill you. |
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[MechE] Ha, never thought of that. DC driving=hell. My son-in-
law almost gave me a heart attack last time I was there.
Whooa there Mel, give old granny a break. |
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Also Lombard Street in San Francisco might not be doable.
(See link). |
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Updated the animation: now one driver on the 'east side'
is doing a right turn and slowing down to be swallowed by
traffic riding in green wave. Another one on the 'west
side' is doing a Michigan left (U plus right) and then
accelerating through yellows to catch green wave in
front.
All turns are going to slow traffic down for sure unless
there's space for turn lanes, but slow green wave speed
makes it possible (while not that safe) to catch up. |
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Thanks for Bigsleep about the 'treat all roads like a
square grid' - addition. This allows the idea be applied to
more grid plans but certainly there are many places
where the whole thing in inapplicable. Boston is a very
straightforward plan compared to many medieval,
mountainous or third world cities. |
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// Anything that causes a partial holdup, ... is going to
result in an expanding pulse of slowness that will
eventually engulf the entire city. // |
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I suppose when traffic everywhere is on the limit of max
flow this will happen. But this is a rush hour situation. I'd
guess more quiet streets will 'recover' from slowdowns,
just like green waves already do. |
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One key question is if whole system could be made to
work as well when busy streets need longer green lights
than quiet ones. |
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And one certain downside is that while riding through the
city slowly but uniterruptedly in zen-mode could be
relaxing, it would also be magnificiently boring. No head-
to-head at lights, no rushing through 'old' greens. You
could just as well take the ******* public transportation. |
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Nice idea - this is kind of the antithesis of another idea ('bidding for greens' - see link) the thrust of which was that as two people approach an intersection at a right angle, the intersection would run a quick auction between the two drivers to sell the right to a green light when they got to the intersection (obviously requiring some comms, and bidding buttons on the steering wheel of the cars). |
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// //During rush hour though, all roads are filled so the
whole system is screwed// Most definitely. // |
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Once this happens the total throughput of the roads
decreases, making the problem that much worse. If a
system was implemented where people were required to
wait in the parking lot until it was their turn, the system
could be operated at the level that provides the
maximum throughput. Therefore the total travel time of
waiting plus driving would be shorter, not to mention
that if it was well orchestrated, a lot of the waiting could
be done with the engine off and computer open... still
inside at your desk. Just be sure to get to your car
before your allocated time slot or else you go back to the
end of the queue. Of course enforcement and
communication with non-networked cars could be
difficult. |
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Does this give enough time for pedestrians to cross? Even
if it does, a pedestrian crossing could block someone from
making a right turn, backing up the cars behind and
ruining the wave. That can be solved with a method
similar to a Pedestrian Scramble. The crosswalks all
come on at once when the lights are red in all directions.
Pedestrians can cross intersections diagonally as well.
Having everyone stop at a light would of course mess up
the wave, so instead have signals visible everywhere, and
every car is required to come to a complete stop.
Pedestrians are then allowed to cross any street at any
location and angle they wish as long as they are out of
the road by the time the signal turns off. At that point
all the cars accelerate at a reasonable pace and the
wave continues. The pedestrian crossing times would
occur infrequently, possibly based on the amount of time
it takes a pedestrian to walk one block. There wouldn't
necessarily be huge crowds waiting at the corners since
pedestrians could cross at any point along the road. |
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// Does this give enough time for pedestrians to cross? //
No, it doesn't give. Good point. Scramble idea sounds
nice. Also normal push button crossing with timing of
exactly two light cycles would work if there's not too
much traffic. Or light cycle times could be doubled if only
even streets had lights (these would need to have a right
of way over the odd streets). |
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Actually if you had lights only for even streets but kept
the fast 20 second light cycle the speed limit could be
40mph, hmmm... |
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Or as my husband says..."Too Bad". |
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