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Old March 18th, 2010, 06:59 PM   #321
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Old March 20th, 2010, 11:35 PM   #322
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Quote:
Originally Posted by ChrisZwolle View Post
A delay of 1 - 2 seconds can significantly reduce the efficiency of a traffic light.
Exactly, and that's why we don't need them (red+yellow). You could be doing the clutch+gear operation on green light. One is a beginner for a short period, so I personally don't see this as a valid argument, and each second lost on red+yellow translates to actual concrete financial losses, which as we all know, are not that tiny.

If getting rid of red+yellow can improve efficiency by a mere 1%, then it's the cheapest was to achieve improvement. It costs next to nothing.
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Old March 21st, 2010, 01:26 AM   #323
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But the red+yellow phase do not influence length of the green phase! If we assume that the light changes to green in "t=0" then you will have the red+yellow from about t=-3 to 0. The only difference is that with 4-phase system you get more information, all the timing constrains remain the same.
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Old March 22nd, 2010, 10:20 AM   #324
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Originally Posted by Fuzzy Llama View Post

But the red+yellow phase do not influence length of the green phase!
OK, to phrase it exactly, adding red+yellow decreases the length of green in the proportion of the entire cycle. If the cycle takes 60 seconds, and red+green is 1 second, that's 1.7%, or not so little.
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Old March 24th, 2010, 01:28 AM   #325
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...like in most of the Europe (I think that the Netherlands are the only European country without "red+yellow" phase).
Italy doesn't have them too.
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Old March 24th, 2010, 02:04 AM   #326
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I could swear they have, at least in Trentino.
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Old March 25th, 2010, 01:31 AM   #327
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Ireland doesn't have the red+yellow phase either.

It does have flashing amber lights (usually in less busy times such as evenings) to permit turns subject to yielding to traffic on the road you're joining
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Old March 25th, 2010, 05:51 AM   #328
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Perhaps TheCat could shed some light on this for me, but does anyone in Canada (Ontario, specifically...) notice that when there's a left turn signal, the green signal is an arrow, but the yellow and red ones are solid? Seems it would make more sense to follow in the footsteps of other places in the world!
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Old March 25th, 2010, 08:23 AM   #329
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Quote:
Originally Posted by LMB View Post
Exactly, and that's why we don't need them (red+yellow). You could be doing the clutch+gear operation on green light. One is a beginner for a short period, so I personally don't see this as a valid argument, and each second lost on red+yellow translates to actual concrete financial losses, which as we all know, are not that tiny.

If getting rid of red+yellow can improve efficiency by a mere 1%, then it's the cheapest was to achieve improvement. It costs next to nothing.
We don't have the Red+yellow phase in Australia, and it shits me to tears.
For some reason the light sequence here is very stretched out so you can easily sit at a set of lights for 3 to 4 minutes before getting a green light. Then the idiots at the front of the line are not ready for the green and it takes them a good 5 seconds on average to start moving. Often if your at the end of the line you will miss the lights by that exact amounts, making you wish you could smack the slow guy at the start around the ears a little. If we had an orange+red phase maybe people would be more ready for the green. At least when I lived in the UK people seemed to move the second the green appeared.
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Old March 25th, 2010, 07:44 PM   #330
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There is no red+yellow phase in Greece either.

I have seen in films that in the early 60's the sequence was "green - yellow - red - yellow - green", but it was re-arranged into "green - yellow - red - green".
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Old March 26th, 2010, 05:45 AM   #331
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Quote:
Originally Posted by rick1016 View Post
Perhaps TheCat could shed some light on this for me, but does anyone in Canada (Ontario, specifically...) notice that when there's a left turn signal, the green signal is an arrow, but the yellow and red ones are solid? Seems it would make more sense to follow in the footsteps of other places in the world!
Well, you're probably referring to the dedicated left turn signals, in which case, you're right - the "left turn signal" sign could probably be made unnecessary by making the red and yellow lights also be shaped like an arrow.

However, those lights are quite rare (especially in the GTA). Most lights only have a permissive green arrow that changes its colour to yellow (older lights in some places in Ontario have a separate yellow arrow, though I've never seen those around Toronto), and there is no need for a red one because it simply disappears, in which case traffic follows the regular solid lights (e.g. yielding to oncoming traffic on green).

Most other places in the world that do have those yellow and red arrows are usually places where dedicated left turn signals are common (like in the west coast of the US, where there is often a light above each lane, and most large intersections have a separate left turn signal).

In Israel, for example, left turn signals are mandatory (i.e., you never have to yield to oncoming traffic on green), the signals are similar to Ontario's dedicated ones (i.e., green arrow and yellow/red solid), but there are fixed arrows above each traffic light, indicating the directions to which the light applies.
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Old March 26th, 2010, 09:53 AM   #332
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I think it makes more sense to have the red/yellow/green arrow phase the same at the solid phase because if it's all uniform, it could be less confusing to colour blind people (I think I read somewhere that's the chief reason the red/yellow/green phase is pretty universal in terms of placement).
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Old March 26th, 2010, 08:52 PM   #333
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Yep, you're correct about that as well, the red is almost universally placed at the top for the reasons you mentioned.

As for your suggestion, the problem with having all 3 left-turn phases (i.e. including the red arrow) with permissive lights is that in that case, if you see a solid green and a red arrow, you will think that you are not allowed to turn left at all (not even after yielding), which is the case with dedicated lights that either permit protected left turns, or prohibit them completely when they are red. This is precisely the reason why permissive lights only have the green and yellow arrow phases, after which the arrow disappears. The arrow gives you a brief period to turn left when all conflicting traffic faces a red light.

In the west coast of the US, as I mentioned, it's common to have dedicated left turn lights (placed in front of the left lane), in which case a red arrow prohibits left turns. Because of that, in some cases, they have a special indication when one is allowed to turn after yielding, such as a blinking yellow (as shown earlier in this very thread, in Seattle) or a blinking arrow.

In Israel, as I also mentioned, this is not a problem because if you get the (rare) signal that has a general solid green without any arrows, you may actually turn left without yielding because in this case the light applies to all directions. Otherwise, there will be a clear arrow placed above the light. As I said, unprotected left turns do not exist there.
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Old March 27th, 2010, 06:58 AM   #334
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Same in Australia. Rarely do you have an intersection without protected right turns. (As we all know, driving is on the left in Australia!)
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Old March 28th, 2010, 05:26 AM   #335
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Quote:
Originally Posted by TheCat View Post
I've always wondered why most tunnels have traffic lights mounted above them,
never quite understood why. Is that in case there is a problem in the tunnel?
And if there is no problem, are they ever red? Because a lot of the time they
are actually on a highway.
If a crash or blockage occurs in a tunnel, it keeps traffic behind from crashing into it.
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Old March 28th, 2010, 05:28 AM   #336
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Quote:
Originally Posted by snupix View Post
What is the red cross that can be seen on French roads on the opposite side of the traffic light?

It looks like this: +
It tells police the traffic light is red. If a car runs the signal, the cop knows he broke the law.

In California, they are small blue dots.
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Old March 28th, 2010, 05:54 AM   #337
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Quote:
Originally Posted by TheCat View Post
That looks quite interesting, because in every one of these pics, there is a dedicated left turn signal, and it seems that you may not turn left unless you have a green dedicated left turn arrow. I really like that.

Is that the case on all streets in these American cities, i.e., are there any traffic light-controlled intersections where you are allowed to turn left while oncoming traffic faces a green light too? Because in Toronto you can, and I really don't like that, and the same goes for other American east-coast cities, like New York.
There are several different types of signals. Engineering is done to determine which kind is used at each signal:

- Exclusively permissive: There are circular signals, but no arrows. Left turns obey the thru signals, filtering through gaps in oncoming traffic.

- Protected/permissive (doghouse): The left turn signal is shared with the thru signal. It has 5 lenses: red ball, yellow ball, green ball, green arrow, yellow arrow. Left turns are protected from other traffic when the green arrow is shown. A yellow arrow ends the protected turn. Left turns are permissive when the green ball is shown alone. They must be made through gaps in oncoming traffic.

- Protected/permissive (flashing yellow arrows): The left turn signal is exclusive to the left turn. The signal has 4 lenses: red arrow, steady yellow arrow, flashing yellow arrow, green arrow. The left turn is protected with a green arrow. Left turns filter through gaps in opposing traffic on the flashing yellow arrow. The steady yellow arrow clears either of the above.

- Exclusively protected: The left turn signal protects all left turns with a green arrow. The red arrow stops all left turns.

Variations:

- The 3-arrow exclusively protected signal can be modified with a flashing red arrow. Each left turning vehicle must stop for the flashing red arrow before turning. Michigan used to use a flashing red ball for this.

- The flashing yellow arrow face can change between the three modes (exclusively protected, protected/permissive, and exclusively permissive) according to time of day or traffic volume present.

- Seattle used to use a flashing yellow ball for permissive left turns.

The use of a left turn phase requires at least one warrant to be met:

- Anything that requires a protected phase (below)
- High left turn vs oncoming volumes
- Left turn accidents
- Poor line of sight
- Excessive left turn delay

The choice of an exclusively protected phase usually requires one of the following to be present:

- High speeds (over 45 mph)
- High traffic volume
- 3 or more oncoming lanes
- Double lane left turns
- Odd intersection geometry
- Poor line of sight
- Prevention of yellow trap (lead-lag signal)
- Left turn accidents
- Heavy pedestrian volumes
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Old March 28th, 2010, 06:29 AM   #338
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More on left turn signals

More on left turn signals

Several different phase patterns are used for left turn signals:

- Single lead: There is a left turn signal facing in one direction. The protected left turn is placed just before the oncoming traffic gets its green.

- Single lag: There is a left turn signal facing in one direction. The protected left turn is placed just after the oncoming traffic gets its green. This is usually limited to places where the cross street is one way, or is the stem of a T intersection, to prevent yellow trap.*

- Dual lead: Opposite approaches receive left turn arrows at the same time. These occur before the thru traffic on the same street is released.

- Dual lag: Opposite approaches receive left turn arrows at the same time. These occur after the thru traffic in the same street is stopped.

- Unsplit Lead-lag (also called split phased): Each leg of the street has its own green with an arrow, at different times. This is usually used with offset intersection legs or small diamond interchanges. Must be used if a lane shares left turn and thru movements and left turn signals are needed.

- Dual split lead: Opposite approaches receive left turn arrows at the same time. But each left turn can end at a different time, when it runs out of traffic. Each thru phase begins after the conflicting left turn ends.

- Dual split lag: Opposite approaches start with thru phases at the same time. But each thru green can end at a different time, when it runs out of traffic. Each left turn phase begins after the conflicting thru phase ends. This must be used with exclusively-protected left turns or flashing yellow arrows to prevent yellow trap.*

- Dual split lead-lag: One approach starts with thru and left phases at the same time. The left turn phase ends when it runs out of traffic or time, and the opposite thru phase begins. When the initial thru phase runs out of traffic or time, it ends, and the remaining left turn begins. Lead-lag is quite useful for two-way signal progression (green wave) where the blocks are short or have unequal length. This must be used with exclusively-protected left turns or flashing yellow arrows to prevent yellow trap.*

These can be assembled with left turn phases on the other street to produce several "quad" left turn signals:

- Quad split lead: Both streets have dual split lead. This is the most common left turn signal in use.

- Dual split lead & dual unsplit lead-lag: The major road is so wide that the cross street left turns can't pass to the left of each other. So each cross street leg gets a separate phase.

- Quad unsplit lead-lag: Each street leg has a separate phase. Usually used with unusual intersection geometry or shared left and thru lanes.

- Quad clockwise: This is similar to quad unsplit lead-lag, except that the order of the legs is different. The legs receive their green lights in a clockwise order (as seen from above). This is an efficient way to move traffic on a small diamond or volleyball interchange with little storage area between the ramps.

Other variations are also used.

* Yellow trap is a dangerous sequence that can cause a crash or trap a left turning driver in the intersection with a red light and flowing oncoming traffic. It happens when both directions on the street have circular greens that allow left turns to filter through gaps in oncoming traffic, and one of the direction's lights turns yellow, but the other direction still has a green.

Preventing yellow trap:
- Use only leading left turns (but watch for phase skip)
- Use only protected phases (lower capacity)
- Use some permissive indication other than circular green (flashing yellow arrow or flashing red arrow). This can be extended beyond the time the circular green can be displayed, allowing permissive turns until the opposing traffic gets its yellow.
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Old March 28th, 2010, 07:02 AM   #339
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There are two reasons we do not have the red+yellow before the green in the US:

- Too many drivers started moving when they saw it. Since it was displayed at the same time as the yellow on the other street, it caused crashes.

- Massachusetts used (and still uses, in violation of federal standards) red+yellow to indicate a pedestrian scramble (all vehicles stop for pedestrians in all directions).

The US Manual of Uniform Traffic Control Devices (MUTCD) has prohibited this display since 1961.

Early signals in the US had very different sequences:

- The first signals had no yellow. They went directly from green on one street to green on the other street. Only two circuits and two bulbs were needed, with the red lights of one street being the greens of the other. One street had the green on top, so it could use the same bulb. A driver on one street would see this sequence:

red
green
red

- The next group had three bulbs, one in each section, with lenses in all 4 directions. One street had green on top, the other had green on the bottom. A driver saw:

red
red+yellow
green
green+yellow
red

- New York City used this sequence, rather than change their signal heads to add yellow. Some of these signals still exist:

red
red+green
green
red+green
red

- When each lens got its own bulb, the following sequence was used in New York:

red
green
green+red
red

- When each lens got its own bulb on three lamp signals, the following sequence was used in some states:

red
red+yellow
green
yellow
red

Since all of the yellows came on together, it saved one signal circuit.

Because of the troubles mentioned above, the federal standards required that only one circular color could be displayed at any time (excluding left turn signals). This produced the cdurrent display:

red
green
yellow
red
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Old March 28th, 2010, 07:15 AM   #340
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I might add that it is illegal in most US states to leave the transmission in neutral while in traffic. Too many drivers coasted in neutral, and then could not recover control of their cars, or rolled back into the car behind while trying to get the transmission out of neutral when the light changed.
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