On congested streets and highways, it is sometimes helpful to use the various lanes for different purposes at different times. For instance,
When circumstances are predictable, changeable lane usage is sometimes managed with signs that explain what movements are permitted at what hours of the day. However, greater flexibility can be achieved with lane-use signals, which are special traffic lights differing substantially from the red, yellow and green dots and arrows found at intersections. Typical lane-use signals employ electric lights, a set of them appearing every few hundred meters along the road. Within a set there is one signal housing, displaying a suitable symbol, over the center of each of lane that is to be controlled. On a street that has reversible lanes, signals for the opposite directions can be mounted back to back. Lane-use signals can be connected to traffic detection devices, reacting automatically to fluctuating volumes throughout the day.
This report often speaks of a clearance interval, a time during which a lane is being converted from one use to another, and at least some motorists will find the latter use more restrictive than the former.
When no clearance interval is in force, the signals are said to be in a long-term state.
Well known are the five lane-use signals specified by section 4J of the Manual of Uniform Traffic Control Devices (MUTCD), published by the Federal Highway Administration of the United States Department of Transportation. All five lights shine steadily; none flashes. The table below lists these signals plus for each a t-symbol, a convenient way to refer to the signal in plain text. Section 5G of this document explains how to derive t-symbols.
| Signal | Meaning | t-Symbol |
|---|---|---|
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| Drivers may not use this lane. | trX |
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| Drivers will soon lose the privilege of using this lane. | tyX |
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| Drivers may use this lane for going straight. If it is the leftmost lane in this direction it may also be used for left turns, and if it is the rightmost lane in this direction it may also be used for right turns. | tgC |
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| Drivers may use this lane only for left turns -- this is a "one-way" turn lane. | tWN |
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| Drivers may use this lane only for left turns, and must watch for oncoming drivers using this lane for the same purpose -- this is a "two-way" turn lane. | tWW |
This scheme does not address several subtleties:
A further problem with the MUTCD system involves an inconsistency with the directions in which arrows point. A white arrow curved downward (tWW) is a warning of oncoming traffic in the same lane, but a green arrow straight downward (tgC) offers a completely different kind of meaning, indicating a lane in which drivers can proceed straight with little risk. Motorists planning to turn left seek either a white arrow curved upward (tWN or tWW), or if such symbol is not displayed over any lane, the leftmost lane with a green arrow straight downward (tgC). In short, some arrows point in the direction a driver is to go, and others point toward the lane a motorist is to use. This irregular arrangement evidently arose as a cross between traffic lights and regulatory signs.
Not surprisingly then, some jurisdictions vary from the MUTCD rules. A flashing yellow X has been used to indicate a two-way turn lane, but because the symbol is not suggestive of its meaning this X could surprise an unfamiliar motorist. Less obscure is a yellow arrow pointing straight downward, which indicates that a lane will soon be closing -- this is a natural way to show a clearance phase after the downward green arrow. Sometimes the yellow arrow points not straight down but rather to the lower left, indicating both that this lane is closing and that drivers should move to the adjoining lane on the left. Feasible of course is its counterpart, pointing to the lower right. These arrows in some areas also appear in green or white.
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| t-Symbol | tyC | tyL | tgL | twR |
This report proposes a new system of lane-use signals ("TFGAP-U"), drawing upon the following features of TFGAP:
These signals can be implemented with light-emitting diodes. Four colors in total are required: red, yellow, green and blue; but because red and green combine to make yellow, installing three colors of LEDs may well be sufficient, as on a computer monitor. The nominal size of these devices might be 750 mm square, which MUTCD 4J.3 specifies as the minimum dimensions for the present two-way left-turn signal (tWW).
Each row in the following table shows an image of a proposed signal, the traffic rules that it represents, and its t-symbol.
| The "Basic Eleven" TFGAP-U Signals | |||
|---|---|---|---|
| Signal | Meaning | t-Symbol | |
 | Flashing Alternately | This is a "two-way" left-turn lane, and it may be used only for left turns. Drivers while preparing to turn must watch for oncoming traffic in the same lane, and when completing their turns must yield to oncoming drivers in other lanes. It is equivalent to tWW. | tGB |
 | Flashing Alternately | This is the clearance interval for tGB. Motorists facing this signal will soon be prohibited from using this lane. No conclusion can be drawn about how oncoming vehicles might eventually be restricted, if at all. | tYB |
 | Flashing | This is a "one-way" left-turn lane, and it may be used only for left turns. Drivers while preparing to turn should not encounter oncoming traffic in the same lane, but when completing their turns still must yield to oncoming drivers in other lanes. It is equivalent to tWN. | tGN |
 | Flashing | This is the clearance interval for tGN. Motorists will soon be prohibited from using this lane. | tYN |
 | Steady | Drivers may use this lane for going straight. If this is the rightmost lane, they may also turn right. | tNG |
 | Steady | This is the clearance interval for tNG. Coming to an end are the privilege of using this lane for going straight, and (if this is the rightmost lane) turning right. | tNY |
 | Left Flashing, Straight Steady | This is tGN and tNG combined: drivers may use this lane to turn left or go straight, and (if this is the rightmost lane) to turn right. | tGG |
 | Left Flashing, Straight Steady | This is the clearance interval for the tNG component of tGG. Drivers will soon lose the privilege of using this lane for going straight, and (if this is the rightmost lane) turning right. Turning left is not affected. | tGY |
 | Left Flashing, Straight Steady | This is clearance for the tGN component of tGG. Drivers will soon lose the privilege of turning left from this lane. Not affected are going straight and turning right. | tYG |
 | Left Flashing, Straight Steady | This is clearance for both components of tGG. This lane will soon be closed. | tYY |
| Steady | Motorists facing this signal must not use this lane. Oncoming drivers might be using it, or it may be vacant entirely. This signal is unchanged from the current MUTCD. | trX |
Contrast between Categories of Signals. Worth noting are the differences among TFGAP-U lane-use signals, the two standard sizes of intersection signals, and a design of lane-control signals to be used on intersection approaches introduced in TFGAP-C. For comparison purposes, here are the signals drawn to scale:
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Key points are:
Pavement Markings. These need not be changed to support TFGAP-U; in particular, the double broken yellow line of MUTCD 3B.3 can still be used between two lanes that carry opposite directions of traffic part of the time.
This hazard, which can occur with current MUTCD lane-use signals, is best explained with an example.
Consider an east-west road having three lanes, of which the north always carries westbound traffic, the south always carries eastbound, and the center reverses depending on the time of day. Motorists are granted the permissive left turn from whichever of these is the leftmost lane for their direction of travel at the time of the turn. Hence eastbound cars will sometimes turn left from the center lane and othertimes the south; westbound vehicles might turn left from the center lane or the north.
As this example continues, an eastbound driver is operating in the right-hand lane because trX is displayed over each of the other lanes; she drives under the sole tgC. Intending to turn into a driveway on the left side of the road, she stops and waits for oncoming traffic to clear. While she is waiting, the lane-use signal over the center lane changes to tgC. This means that, although this motorist has made no mistake, she is now in the wrong lane to make her left turn.
What leads to such a crash is that the center-lane motorist going straight is relying upon up-to-date information, and the right-lane driver turning left has out-of-date information. In general, motorists respond to changing traffic signals at varying rates, and this is one of the reasons that clearance intervals are universally implemented on intersection signals. The author believes that lane-use signaling can as well benefit from a fuller offering of clearance intervals, and that is the key motivation for TFGAP-U. The tyX for clearance specified in the MUTCD is often not sufficiently precise: for one thing, the it cannot indicate that left-turn privileges are about to end when straight privileges are going to continue.
Here is a TFGAP-U sequence of signals, for eastbound traffic, to handle adding a lane to the left:
| North Lane | Center Lane | South Lane | Comments |
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| Eastbound drivers may turn left from the right-hand (south) lane. |
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| Eastbound drivers are warned that turning left from the right lane will soon be prohibited. This is a yellow clearance interval. |
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| Eastbound drivers may not turn left from any lane. This is a red clearance interval. |
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| Eastbound drivers may turn left from the center lane. This step is optional, but by delaying center-lane tGG it promotes safety. Because motorists heading straight are not yet permitted in the center lane, traffic there will be lighter than otherwise. Hence a right-lane driver who is relying on obsolete information and attempting to turn left wil be somewhat less likely to crash into a center-lane car than she would if the center lane had a tGG signal and the resulting straight traffic. |
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| Eastbound drivers may use the center lane for turning left or going straight. |
Some readers may be disturbed that during the red clearance interval, the left turn from any lane is prohibited for eastbound traffic. This limitation is in fact required for safety, and TFGAP-U is able to communicate it. If the left-turn privilege were offered to drivers continuously, then its transfer from one lane to another would have to occur instantaneously. Since some drivers would react to the new state of affairs faster than others, their disparate opinion on what lane is to be used for what movements would lead to crashes, as in the example above.
This scenario involves shifting the left-turn lane to the left, but a similar hazard arises when the left-turn lane is being shifted to the right. Again, the surest way to achieve safety is to prohibit left turns for a brief time.
In some cases, lane usage is controlled not by signals but instead by signs that list what movements are permissible at what hours of the day. It is possible to schedule a no-left-turn clearance interval for each lane shift, but this can lead to complicated signs. Worsening matters is that automobiles' clocks and drivers' watches routinely deviate from official time, being ahead or behind by any number of minutes. A long clearance interval will mitigate the danger from this inaccuracy, at the expense of making travel less efficient.
5A. Option: Red Arrows. Further signals can be devised by substituting, in some of the basic eleven signals, a red arrow in place of a yellow one. This is either to emphasize that a certain movement is prohibited, or else to effect an explicit red clearance interval following the normal yellow one. The following table shows what some would look like.
| Signal | t-Symbol | |
|---|---|---|
 | Left Flashing, Straight Steady | tGR |
 | Left Flashing, Straight Steady | tYR |
 | Steady | tRG |
 | Steady | tRY |
 | Steady | tRR |
 | Red Steady, Blue Flashing | tRB |
Simple examples of red clearance intervals for a lane are the sequences tGG-tGY-tGR-tGN and tGG-tYG-tRG-tNG. Also possible are more elaborate patterns such as tGG-tYG-tYY-tRY-tRR-trX. Signal programmers enjoy this great flexibility because the timing of left-turn clearances is independent of the timing for straight clearances.
Although red arrows are optional in TFGAP-U, they are mentioned at this point because their potential existence influences the reasoning given in the next three sections.
5B. Rationale: Flashing and Steady. As part of TFGAP, the green and yellow left-turn arrows must flash in order to indicate the permissive nature of that movement, as drivers must yield to pedestrians and oncoming motorists. On the other hand, the red left-turn arrow if used should be steady because it is displayed to prohibit the left turn; a flashing red turn arrow is not correct except in the rare case that drivers are allowed to turn only after stopping first and then yielding. Unknown is what purpose a flashing straight red arrow might serve.
Straight movements are automatically protected, so their arrows in green and yellow colors are on steadily. Although drivers in the rightmost lane are governed by a straight signal, they can by default turn right after yielding to other traffic, principally pedestrians.
Section 5D of this report explains why the blue arrow in the two-way left-turn signal should be flashing. This opens the question of whether the two arrows within each of tGB and tYB should flash simultaneously ("in phase") or alternately ("out of phase"). A point in favor of the alternating flash is that the two-way left-turn signal, containing two complete left-turn arrows, is somewhat crowded. Displaying them one at a time leaves no doubt which stem goes with which head, and this is the option chosen.
5C. Rationale: Hollow and Solid. In the case of ordinary intersection lights, either of the shapes (arrow or dot) might appear in any of the colors (red, yellow or green). However, motorists with deficient color vision can usually infer the color of an intersection light from its position in the assembly: normally red is on the top or left, yellow in the middle, and green on the bottom or right. Such positional cues are difficult to implement in lane-use signals without entailing huge displays.
For this reason the red, yellow and green TFGAP-U arrows of different shapes: the green arrow has a hollow stem with solid head, the yellow has a hollow stem with hollow head, and the red has a solid stem with hollow head. Meanwhile, the TFGAP-C signals are solid in both stem and head.
Discrepancy in target value is addressed by giving hollow stems and heads greater overall dimensions than their solid counterparts, meaning that the numbers of pixels lit in the hollow and solid cases will be more nearly equal. To illustrate are the following large-scale overlay diagrams of lane-use signals:
Below are lane-control signals, normally white but here shown in gray for color contrast, on top of yellow lane-use signals:
5D. Rationale: The Blue Arrow. Most of the time, the blue arrow in this lane-use signal system represents a flashing green left-turn arrow presented to the oncoming traffic that is operating in the same lane as the motorist who sees the blue arrow. At other times, a relatively brief clearance interval is in force, and the blue arrow instead would stand for an arrow that is flashing yellow, steady red, or nonexistent altogether.
This proposal opts for the color blue because under TFGAP, blue is the color of warning lights, and drivers certainly need to be alerted when there is the prospect of oncoming vehicles in the same lane. Red, yellow and green are already used to show directions in which drivers can at least sometimes proceed, as precedented with intersection signals. Meanwhile, white and orange are taken by traditional pedestrian signals, and TFGAP for pedestrians further employs violet. Blue is one of the few colors that remain to represent a direction in which it is impossible (not merely unlawful) for a motorist to go.
Among the reasons for this blue arrow to flash, rather than be steady, is that with the rarest of exception highway warning lights have always flashed, and it is best to satisfy driver expectation. A second reason for the blue light to flash is that the green turn arrow signal presented to the oncoming drivers itself flashes; hence the signal that warns of the hazard is made more similar to the signal that causes the hazard. By that line of reasoning, the blue arrow stem should be hollow to match the green, both heads should be solid, and TFGAP has adopted this approach.
During the first and perhaps only clearance interval for oncoming motorists, their flashing green solid-head arrow will change to a flashing yellow hollow-head arrow. Should the head of the blue arrow make the parallel change from solid to hollow at this time? No, that is a needless complication. The blue arrow should be left alone because there continues to be risk of oncoming cars in the lane, and that is the full extent of information that drivers need; there is no advantage in notifying them that a hazard will soon be eliminated, because some drivers will reduce their level of caution prematurely. A similar principle has long been embodied at intersection lights in the United States, where a driver waiting at a red light receives no official indication that it is about to change to green.
By the same token, the blue arrow should continue to be displayed flashing with hollow stem and solid head during any further intervals that clear oncoming traffic from the two-way left-turn lane. This includes periods when oncoming drivers see either a steady red left-turn arrow (which has a solid stem and hollow head) or perhaps no left-turn arrow at all. Once all clearance intervals from the two-way left-turn configuration have expired, the blue arrow should be extinguished.
In tabular form, here is a highly articulated example of converting a two-way left-turn lane into one-way:
| Step | Signal for Eastbound Traffic | Signal for Westbound Traffic |
|---|---|---|
| 1. Long Term | Flashing Alternately | Flashing Alternately |
| 2. Yellow Clearance (required) | Flashing Alternately | Flashing Alternately |
| 3. Red Clearance (optional) | Flashing Alternately | Red Steady, Blue Flashing |
| 4. Implicit Clearance (optional) | Flashing Alternately | Steady |
| 5. Long Term | Flashing | Steady |
5E. Option: Narrow Housings. On a limited access highway where turns are not possible, the only arrows necessary are straight ones. They can fit into a narrower device, perhaps 500 mm wide instead of 750 mm.
5F. Option: The Right-Turn Arrow. In the rare case when this is installed it will be analogous to the left-turn arrow: steady when red, but flashing (because of yielding to pedestrians) when green or yellow. A signal enclosure that needs to contain only a straight arrow and a right arrow can be the mirror image of a basic eleven signal, posing no problem. On the other hand, if a single lane requires signaling of left turns in addition to straight movements and right turns, the three necessary arrows will hardly fit into a 750 mm square device, and something else will have to be used.
Here is an application of the right-turn arrow. A ramp, carrying heavy traffic that must merge onto a surface street, approaches the surface street from the right. Approximately 500 meters upstream of the junction, lane-use signals on the surface street terminate use of the right-hand lane for drivers wanting to go straight; those motorists, who see a steady red straight arrow, must move a lane to the left.
Although this lane shift will clear the right lane to ease the merge for traffic coming in from the ramp, the red arrow by itself would bar those surface-street motorists who need to turn right onto a side street at some point before the merge. This is because the straight arrow over the right-hand lane also by default regulates right-turn traffic.
When installed over the right lane, the signal pictured below will solve this problem; it says that the lane underneath must not be used for driving straight, but may be used for turning right.
| Signal | t-Symbol | |
|---|---|---|
 | Straight Steady, Right Flashing | tNRG |
5G. The t-Symbols. These tokens always begin with a minuscule 't', which may be thought of as standing for "text". They are designed to be brief and mnemonic, and to give few false hits in a case-sensitive text search.
| When Second Letter is Lower Case | When Second Letter is Upper Case |
|---|---|
| Generally, these symbols tell drivers what lane to use.
Second letter:
| Generally, these symbols tell motorists in what direction to drive.
Second letter:
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TFGAP-U uses only one signal from the left column, the trX.
Not all combinations of signals can be shown simultaneously to the two opposing directions of traffic in a lane. In fact, only a few pairings are safe. Here they are:
| Signal for one direction | Safe signals for the other direction |
|---|---|
| tGB, tYB | tGB, tYB, trX |
| tGN, tYN | trX |
| tNG, tNY | trX |
| tGG, tGY, tYG, tYY | trX |
| trX | Anything |
Some of these combinations merit comments. The caret (^) will denote the two signals for opposite directions in the same lane of traffic. In order to make the examples concrete, the t-symbol before the caret will be intended for eastbound traffic, after the caret for westbound. These pairings drop the 't' character from the westbound t-symbol as redundant.
trX^rX. Some dispositions may have a long-term empty lane separating opposing directions of traffic in the interest of safety. As an example, a seven-lane street could have two lanes in one direction, four in the opposite direction, and one vacant lane between them. Moreover, vacant lanes are indispensible for red clearance during direction-change operations.
tGB^rX and trX^GB. While tGB and trX are long-term symbols, the tGB^rX and trX^GB combinations are not useful except for clearance as in this sequence:
The reason for this limitation is that tGB^rX effectively prevents westbound traffic from turning left from any lane. This is because tGB^rX specifically informs eastbound drivers that westbound left-turn traffic will be in the same lane that eastbound left-turn traffic uses, but then it prevents westbound traffic from actually using that lane. If westbound traffic is instead directed to turn left from some other lane, eastbound drivers may be surprised, with crashes resulting. The tGN^rX and tGG^rX signals do not incur the same problem, because they make no statement to eastbound traffic about from what lane which westbound traffic might be turning left, beyond saying that it is not a lane that eastbound vehicles are using.
Lane control for intersection approaches has been moved.