A metering signal is a special traffic light, used in areas of heavy volume, controlling the rate at which vehicles pass a certain point. By rate we mean the number of vehicles per minute, not their speed as measured in kilometers per hour. A prominent usage is on entrance ramps to freeways, which often operate more smoothly when merging vehicles are evenly spaced. In other cases, when demand for a freeway simply exceeds its capacity, metering is installed to restrict the overall number of vehicles entering the highway; this encourages drivers to find alternate routes. Metering can also be helpful on roundabouts when some approaches have far more traffic than others. These applications are discussed later.

A metering signal should not be installed at an intersection, because it does not assign right of way. Indeed, it must be placed a significant distance away from intersections or merge points. More specifically,

• Its location must be far enough from any upstream intersection that queued vehicles do not overflow into that intersection. Because metering is typically implemented in conjunction with vehicle detection, engineers can program the controller to alter the timing whenever the queue comes close to blocking the intersection.
• The metering signal must be installed far enough from a downstream intersection or merge point in order that drivers will not misinterpret the metering signal as controlling right-of-way there. Also, drivers must have enough distance to accelerate if merging onto a high-speed road.

Normally a meter is active only during rush hours, and at other times vehicles are permitted to proceed undelayed. A sign reading ONE CAR PER GREEN is often posted, helping drivers recognize the special purpose of the signal. Some jurisdictions prefer the more precise term VEHICLE instead of CAR.

A platoon is a cluster of vehicles traveling in the same direction at about the same speed. It may be one or several lanes wide, but this report concentrates on the one-lane version.

2A. Good Platoons. In the right situations platoons can increase traffic throughput.

In the classical example, the capacity of an all-way stop intersection (first come, first serve) will likely be increased by converting it to a red-yellow-green signal. Every red light builds a platoon, and on green lights cars pass through in batches and not individually as with the all-way stop. Reassigning the right-of-way from one approach to another involves dead time as cars clear the intersection. The red-yellow-green signal increases efficiency because it reduces, compared to the all-way stop, the number of such reassignments per minute.

Further, in a network of city streets with signals at most intersections, planners often implement a centralized controller for the whole system of signals. Platoons, once formed, can retain their integrity for many blocks and will rarely need to stop if lights can be suitably timed. At a typical intersection, the north-south street ideally receives a green light during the gaps between platoons traveling on the east-west street.

2B. Bad Platoons. On the other hand, platooning sometimes leads to problems.

Merge Ramps. A familiar example involves an ordinary diamond interchange in a congested urban area, with red-yellow-green traffic lights controlling both intersections. The effect of the signals is that cars are fed onto the merge ramps in platoons, and (without special treatment) will merge in platoons. Not only the merging drivers, but also those in the right-hand lane of the mainline must cope with certain difficulties.

• Merging drivers. These motorists must find a gap in mainline traffic, and move into it.
  • When an isolated vehicle tries to merge onto a crowded freeway, its driver has considerable latitude in adjusting his speed to synchronize his car with a gap in traffic. With no vehicle on the ramp in front of him, he can accelerate sharply if necessary; and with no vehicle on the ramp in back of him, he may brake heavily if required.
  • In contrast, when a platoon of cars tries to merge each driver's acceleration is limited by the speed of the vehicle in front of his. Too, a driver may be reluctant to substantially decrease his speed due to the risk of inducing a rear-end collision. Consequently, he may effectively be forced to merge into a dangerously small gap.
• Mainline drivers. Motorists in the right lane of a crowded freeway frequently try to assist merging traffic. One method is for them to change lanes to the left, creating a larger right-lane gap; but in heavy volume there may be no vacancy in the next lane. In that case,
  • When only one vehicle is trying to merge, the right-lane driver may be able to enlarge a gap by tempering his speed. He can accelerate once he concludes that a merging driver will be entering behind him. Similarly, the right-lane driver can decelerate once he determines that a merging driver will be entering in front of him.
  • Unfortunately, when a platoon of cars is trying to merge, any speed change that helps one merging motorist may hurt another, and the right-lane driver has little power to improve the situation.

A metering signal that is well calibrated will inject vehicles onto the freeway at a steady rate, one at a time. Hence, merging drivers will enjoy more leeway in speeding up or slowing down to find a gap, and motorists in the right lane of the mainline will have more opportunity to adjust their own speed to ease the merging of one car, rather than several closely spaced.

Roundabouts. In this variety of circular intersection there is the risk that heavy traffic on one approach may monopolize the circulating roadway, preventing entry for motorists on other legs. Because roundabouts operate at much lower speeds than freeways, the safety consequences of this problem are much smaller; even more so since approaching drivers may sensibly come to a full stop and wait when there is no gap in circulating traffic. (By contrast, stopping on an expressway merge ramp is generally very dangerous.) Still, motorists who have been "stuck" for a long time waiting to enter the roundabout will become frustrated, and may be tempted to try risky movements to get going again.

A metering signal installed on a high-volume roundabout approach can correct this problem by briefly detaining drivers on the high-volume entrance. This gives motorists on the other approaches a chance to enter the circulating roadway.

Fine Metering. This breaks up platoons for easier merging onto freeways, and is characterized by the ONE CAR PER GREEN sign. A disadvantage is that cars in the queue may have to stop and start many times while waiting; this wears on brakes, clutches, and drivers' patience. When fine metering is used on a multi-lane approach to a single-lane merge ramp, each lane in turn gets the green signal.

Fine metering divides platoons into individual vehicles. Although installed for a completely different purpose, all-way stop control at an intersection has some tendency to fine-meter traffic because of the delays incurred by requiring every vehicle to stop.

A variant allows not one but two vehicles to pass on each signal; the aim is to increase throughput, but this gives drivers less flexibility at the merge point. Also there is a difficulty of enforcement, as follows.

Suppose the queue at a red metering signal is headed by a subcompact auto, a large tractor-trailer, and a van. The driver of the van cannot see the subcompact because the truck blocks his view, so the van driver believes that he is second in the queue when in fact he is third. After the light turns green all three vehicles go. A nearby police officer observes this, and tickets the van driver for violating the two-per-green limit. Of course, police personnel can certainly be educated about this visiblity problem, and they can strive to refrain from citing such view-blocked motorists. However, an observer parked at the side of the road cannot always determine exactly what someone else can or cannot see, and concerns about inconsistent enforcement will be inevitable.

Coarse Metering. This pauses the flow of traffic periodically, so that motorists on side roads downstream can more easily yield and enter the main road. When applied to a roundabout, traffic on one approach is held so that motorists on other approaches can move into the circulating road. In coarse metering, cars are permitted to proceed several at a time; the light may operate on a simple timer, or it may be attached to vehicle-detection equipment that counts the passing cars. Such number will be approximate, because detectors are not perfect and there is no way to predict exactly how many drivers will squeeze through a yellow light.

Coarse metering divides large platoons into small platoons, or gathers individual vehicles into small platoons. An ordinary red-yellow-green signal at an intersection can incidentally result in a significant level of coarse metering, especially if few drivers make turns.

With either kind of metering it is possible to have a bypass lane, which privileged traffic such as high-occupancy vehicles may use. Motorists in the bypass lane have either no signal or a continuous green one, and need not stop regardless of the signals displayed to the regular lanes. An alternative to the bypass lane for privileged traffic is the priority lane. Although this lane does have a metering signal, it operates on a schedule more favorable than the signals for the regular lanes, hence priority motorists enjoy a shorter wait.

4A. Four-Phase Metering. Fine metering is in its greatest generality a four-state operation, and this report proposes a four-section signal in which exactly one section is lit at any time.

State		Signal
Metering Off	All vehicles may proceed.	Steady Green Dot
	Vehicles should stop, but may proceed if stopping is impossible.	Steady Yellow Dot
Metering On	One vehicle may go; others must wait.	Steady Green Numeral One
	All vehicles must wait.	Steady Red Dot

Three versions of the signal appear below. The two-by-two pattern is quite unlike any of the usual arrays of intersection signals in MUTCD 4D.16, and will imply to drivers that this signal has a special purpose. Still, this arrangement respects certain customs:

• Green is below red and yellow as in a standard vertical intersection signal.
• Red is to the left of yellow as in a standard horizontal signal.
• The green dot is next to the yellow dot that signals its clearance phase.

The two linear versions may work better in cramped spaces. Within each, the green numeral is next to the red dot, because the signal will cycle between those two lights hundreds of times per day. Other cycling will occur only a few times per day, typically at the beginning or end of each rush hour.

To avoid confusion, the numeral one must be sufficiently distinct from the straight-through arrow of intersection signals, below left; and the white bar of transit signals (MUTCD 10D.07), below right. Subtleties of geometric design are key to success.

Also we recommend that each light be in the larger of the two standard diameters, 300 mm. Since the 200 mm fixture is regarded as too small for an arrow by MUTCD 4D.15, it would presumably be insufficient for a numeral, also. (But see section 7C below for a possible exception.)

4B. Two-Phase Metering. Where metering is always in effect, a simpler signal can be used:

Whether two-phase or four-phase, the green numeral is always next to the red dot.

Two-phase metering has been used for decades at traditional toll plazas. The driver at the booth pays his money, receives a green light and goes. The next driver pulls up to the booth and stops even if the light still happens to be green, because he has not yet paid his toll. Toll plazas operate successfully with a green dot instead of a numeral or other special signal, because the context of the toll plaza makes it clear that every driver must stop at the booth. Many toll booths lack a yellow signal, demonstrating that no clearance interval is needed in one-car-per-green operation.

4C. Sequence of Signals. The useful signal changes are as follows:

Change	Metering ...	Message
Green Dot to Yellow Dot	will soon begin	Vehicles must stop if possible; otherwise they may proceed.
Yellow Dot to Red Dot	begins	Vehicles must wait.
Yellow Dot to Green Numeral		One vehicle may proceed.
Green Numeral to Red Dot	continues	Vehicles must wait
Red Dot to Green Numeral		One vehicle may proceed
Red Dot to Green Dot	ends	All vehicles may proceed
Green Numeral to Green Dot

Safe, but of little use are the changes from green numeral to yellow dot, red dot to yellow dot, and yellow dot to green dot. On the other hand, a direct change from the green dot to the green numeral or red dot could induce panic stops.

With most traffic signals, a clearance interval is required when a light goes from green to red. The change from green numeral to red dot as presented in this plan is an exception:

• No clearance is needed for safety, because there is no conflicting traffic.
• No clearance is needed for preventing panic stops. The numeral will always last long enough for the first driver in the queue to get through, at least if he is attentive. After that, the second driver must stop when he reaches the stop bar, due to the one-car-per-numeral restriction, even if the signal is still green.

Engineers often try to use a three-dot signal for fine metering, but then additional signs or signals become necessary to fully inform motorists of the arrangement. (One example is a sign, bearing a yellow signal, with the message RAMP METERED WHEN FLASHING.) This is at odds with an important principle: if one traffic device cannot be fully understood without reference to other traffic devices, then the devices probably need to be redesigned.

5A. The steady green dot to indicate that metering is off is consistent with this symbol's meaning at intersections: drivers may proceed straight through without delay. When displayed at an intersection the green dot permits a turn, but drivers at a metering station are not likely to attempt any turn because there is no crossroad. Here are some alternatives to the steady green dot, and why they were not chosen in this proposal:

• Shutting down the signal entirely during non-metering times is risky. Some drivers when approaching a dark signal will assume that it is broken and come to a complete stop, by analogy with the necessary procedure at intersections. Other drivers will proceed, because they understand that a dark metering signal means to keep going. With some motorists stopping and others going, there is risk of rear-end collisions. To address this conflict may be installed a sign informing drivers that they should not stop when the signal is unlit. However, this sign would likely be in addition to a ONE CAR PER GREEN sign, and any traffic device that requires multiple explanatory signs really needs to be redesigned for simplicity. Moreover, drivers who learn that metering signals are to be ignored when dark may cause crashes by extending this principle to intersection signals, which are dark only when malfunctioning and must never be ignored.
• A flashing green dot may confuse those motorists who when they see the ONE CAR PER GREEN sign believe that they must somehow synchronize their car's position with the on-and-off cycles of the light. Also, in TFGAP a flashing green signal indicates the need to yield, which is irrelevant to metering.
• A flashing yellow signal, under traditional traffic rules, means that motorists may proceed but ought to use caution. Yet because no particular hazard exists at a metering station, a caution light amounts to a false alarm. Another matter is that a clearance interval, when metering is about to start, would presumably be indicated by altering the display from flashing yellow to steady yellow, but this difference might be too subtle for motorists to notice in a span of merely "3 to 6 seconds" (MUTCD). Although the duration of clearance could be extended, motorists who regularly drive through the metering station would then learn that the steady yellow signal lasts an exceptionally long time, and they would be tempted to run it.
• A flashing yellow signal, under a fully-implemented version of TFGAP, signals clearance of a flashing green phase. Since metering does not use flashing green, it cannot use flashing yellow.

5B. The steady yellow dot for the clearance phase is the natural counterpart to the steady green dot. It accords to the principle that, except for color and duration, the yellow signal for a clearance phase should be identical to the green signal that it follows.

5C. The steady green numeral one indicates that exactly one vehicle may pass. The light must be steady because if it were flashing, some drivers would infer that one car could proceed on each on-and-off cycle. The signal is green because green means go. The traditional dot and arrow shapes will not work because they say nothing about the one-car limit.

Upon seeing the numeral, the unfamiliar driver might ask "one what?"; hence a more specific educational sign, such as the following, needs to be posted on the left of the display until the meaning becomes well known.

To be more specific, THIS LANE can be replaced by LEFT LANE, CENTER LANE, et cetera.

5D. A steady red dot means, as always, that drivers may not proceed straight. There should be no concern about motorists attempting the right turn on red, because there is no crossroad onto which to turn, and the stopped motorists ought to be able to see that. Some readers might prefer a red numeral instead of dot by analogy with the increasing trend, at intersections, of using a red arrow instead of dot in a left-turn signal that employs green and yellow arrows. Here are two responses to that viewpoint:

• The educational signs shown above intend to communicate "one car per green numeral", but some motorists may think they mean "one car per numeral" (of any color). These motorists might try to proceed not only when the light changes from red numeral to green numeral, but also when it changes from green numeral to red numeral. Although this misunderstanding should not cause a collision, it would roughly halve the benefit of metering.
• The red left-turn arrow has two main functions:
  • Prohibiting the left turn when a green signal is presented to other motorists, on the same approach, who are planning to go straight or turn right.
  • Emphasizing, to those approaching drivers who are still selecting a lane, that a certain lane is for left turns only.
  At a metering station, the segregation of drivers into lanes is of significance only if priority lanes are established for certain vehicles -- motorists will certainly not be making turns. To convey the pertinent lane-restriction information in a traffic signal of only 300 mm diameter is nearly impossible, and signs become inevitable. This leaves the red light to bear no message except "stop", and we have chosen the most familiar shape of all for this purpose: the dot.

Coarse metering, or three-phase metering, is altogether simpler than its fine counterpart.

State	Signal
All vehicles may proceed.	Steady Green Hourglass
Vehicles should stop, but may proceed if stopping is impossible.	Steady Yellow Hourglass
All vehicles must wait.	Steady Red Hourglass

The hourglass, a novel figure that suggests some sort of timing or waiting, is intended to inform motorists of the purpose of the signal. Still, a METERING SIGNAL sign will be needed during the educational period. The METERING SIGNAL sign should stay up permanently if, instead of using the hourglass shape, a highway department opts to implement all three sections as dots.

The sequence of signals is the same as with the traditional three-dot traffic light: green, yellow, red. Unlike fine metering, coarse metering requires a yellow clearance phase because all motorists who arrive in time may proceed, and a sudden change from green to red would lead to panic stops. The display can be horizontal or vertical:

A green signal does not explicitly indicate whether metering is on or off, but it does not need to: the meaning of green is the same either way. On the other hand, metering is patently in effect when the signal is yellow or red.

7A. The Problem. As motorists are approaching a red light or stop sign, they routinely pass the the stop bar painted on the street when they bring their vehicles to a halt. This is often due to carelessness, although in other cases water on the road may obscure the bar, or its paint may be heavily worn. As an illustration, many drivers at an intersection will pull up to the very edge of the lane for cross traffic, blocking pedestrians who want to use the crosswalk.

This deficiency explains why highway departments so frequently install traffic signals on the far side of a intersection -- in other words, beyond the cross street from the motorists who must obey them. Even drivers who have edged into the intersection can still see the lights thus placed. In some instances however, signals do need to be installed on the near side, as when the intersecting roads are nearly parallel. Then the following scenario can unfold:

1. A driver approaching the intersection sees a red light.
2. Ignoring the stop bar, he makes a decision about where he is going to stop.
3. He stops.
4. Then he realizes that he has gone past the stop bar, is beyond the signal's cone of visibility, and will not be able to see when the light turns green.

Such overshooting can also happen at metering signals, and is indeed likely to, because there is no crossroad defining the last point at which a driver can stop without major risk of right-angle collision. Metering signals must be aimed so that motorists can see them from a hundred or more meters away. Their cone of visibility is often so narrow, and so nearly horizontal, that a driver who has pulled to within a few meters of the signal can no longer see it even though he has not yet passed the light fixture.

At one- and two-lane metering stations the problem can be mitigated by erecting, a few meters upstream of the light, wayside signs reading STOP HERE ON RED with clarifying arrow. Yet this may not be enough to prevent all violations. Moreover, when there are three lanes, a driver in the center lane may not see the signs due to vehicles in the outside lanes blocking his view. While any motorist should be able to see the STOP HERE ON RED sign when it is installed overhead, a short distance ahead of the signal, the risk now becomes that the sign will block view of the signal for those other drivers who are farther upstream, perhaps 100 meters.

7B. A Solution. The continuing advancement of optical technology may someday yield a lens that will distribute light in an irregular cone suitable for this situation, but for now engineers are tackling this problem with auxiliary signals.

At a one-lane metering station for instance, a wayside pole holds a metering signal at the usual height, and it is aimed at approaching cars that are still dozens or hundreds of meters away -- this is the main signal. On the same pole but lower is mounted another metering signal, and it is aimed toward the driver of a car that has pulled almost even with the pole. The idea of course is that unless the motorist has passed the signal entirely, he can see either the main signal or the auxiliary.

Auxiliary signals will also work for two-lane metering stations, but three-lane applications require examination. If auxiliary lights are being mounted on wayside poles, then the center lane's auxiliary signal would presumably have to share a pole with the auxiliary for one of the outside lanes. Recall that in fine metering, green is given to one lane at a time, so each lane requires a separate signal. With some complication, signs could sort out which auxiliary signal belongs to which lane, but even then a vehicle in the outside lane may block the center-lane driver's view.

Note that for the center lane, the main signals will almost certainly be mounted on a mast overhead. This suggests installing an auxiliary signal, aimed rather downward, on the mast beneath the main signal -- such arrangement would give an approximation to the irregular-cone lens mentioned at the beginning of this section.

In the side view above, three motorists moving from the reader's right to left are on the approach to an overhead metering signal with main and auxiliary lights; the signals are currently red. Each cone of visibility is shown in light pink, with their overlap in dark pink.

• Driver A, who is still some distance away, can see the main signal.
• Driver B, who is waiting at the stop bar for green, also has the main signal in view.
• Driver C, who has overshot the stop bar, must rely upon the auxiliary signal.

7C. Some Details.

While the main signal should have lenses of diameter 300 mm, the 200 mm size should suffice for the auxiliary because any driver relying on it will necessarily be very close to it. For the same reason, a low-brightness lamp may be in order, so as not to blind the driver with glare; this is a critial factor at nighttime.

The cone of visibility for the auxiliary signal might be wider than the cone for main signal, because the auxiliary is intended for drivers of vehicles that are in a variety of incorrect locations near the light source. The two light cones should be aimed with some overlap for when signals sway in the wind.

In four-phase metering, an open question is whether the auxiliary signal should include all four sections:

• The importance of the red dot and green numeral should be obvious, as each is used during every metering cycle.
• The green dot is necessary when a driver happens to be stopped in the overshoot zone just when metering ends. Otherwise, after his red dot he sees only a dark signal, and he has no idea that he can proceed. After perhaps thirty seconds of unmetered service the auxiliary green dot can be extinguished, because by then no drivers should be stopped in the overshoot zone; instead, all will be proceeding according to the main green dot. When LED signals are used, the green dot and green numeral might feasibly reside in the same section.
• The auxiliary yellow dot (AYD) is debatable. On the one hand, the AYD is useful when an approaching motorist sees the main yellow dot, decides to stop, and passes the stop bar. Because drivers often make quick stops on yellow, they are especially likely to overshoot. Were the AYD to be omitted, such drivers would be presented a dark signal, which is undesirable. On the other hand, this problem can happen only at the beginning of metering, which would occur perhaps three times a day. Further, a motorist in this situation would be uninformed for no more than six seconds, and likely much less, because a red dot or green numeral would soon appear. If the dark signal does confuse the driver, in his uncertainty he will either wait or go, and both actions are safe. Hence omission of the AYD might be justified for the cost savings.

In three-phase metering, the auxiliary signal should certainly include all three sections. In particular, the yellow light is important because when metering is in effect, it is lit frequently, perhaps once per minute.