Traffic volume in the United States is continually growing, and roadways cannot always be expanded to handle the load; thus a more efficient use of the existing pavement becomes critical. Additionally, many planners are trying to make roadways more hospitible to pedestrians, giving travellers a viable option to cars for short trips.

Toward this end, TFGAP seeks to refine the set of messages -- the vocabulary -- that traffic signals in the United States can convey. Of course, a balance must be struck: signals that are very complicated may offer a broad vocabulary, but excessive detail will merely overwhelm both motorists who must already be watching the road and its hazards, as well as pedestrians many of whom are children who cannot grasp elaborate signaling systems.

An ironic inconvenience is that the most complicated signals are required in precisely those locations where travelers already have the largest amount of traffic to monitor. Moreover, signals by their nature can change from moment to moment, which fact burdens motorists who might be required to analyze a signal multiple times as they are moving toward it. Therefore, traffic lights must be simple.

Beyond that, we must have a system that is highly compatible with current practice in order to prevent the task of educating travelers, and the expense of converting hardware, from being overwhelming.

Two essential terms throughout this discussion are permissive, which means that a traveler must yield in making some move; and protected, which means that a traveler should not have to yield. The most noteworthy new signals in TFGAP are flashing green and yellow signals indicating permissive moves to drivers, and violet signals indicating permissive moves for pedestrians.

These reports presume that motorists normally drive their vehicles on the right-hand side of roadways. Readers in areas where drivers routinely operate left of center should of course exchange "left" and "right" throughout this document.

2A. General. Broadly, the TFGAP signals mean this:

Signal		Meaning to Motorists	Meaning to Pedestrians	Meaning for Transit
Green	Steady	Protected movement
	Flashing	Permissive movement
Yellow	Steady	Clearance of protected movement
	Flashing	Clearance of permissive movement
Red	Steady	Prohibited movement
	Flashing	Permissive movement after stop
Blue	Steady	not used
	Flashing	Warning	Warning
White	Steady	Lane control	Protected movement
	Flashing	Clearance in lane control	Clearance of protected movement
Violet	Steady		Permissive movement	Locally defined
	Flashing		Clearance of permissive movement
Orange	Steady		Prohibited movement
	Flashing	not used

In order to increase compatibility with traditional signals, the design of TFGAP contains an inconsistency. Some clearances are indicated by a change of color, others by a shift from steady to flashing:

Signal prior to clearance	Signal during clearance ...
Steady green	changes to yellow	remains steady
Flashing green	changes to yellow	remains flashing
Steady white	remains white	changes to flashing
Steady violet	remains violet	changes to flashing

Many signals may be tabulated as follows:

Travelers	Signal	Normal	Clearance	Comments
Motorists	Steady green and yellow			Least restrictive
	Flashing green and yellow
	Flashing red		None
	Steady red		N/A	Most restrictive
Pedestrians	Steady and flashing white			Least restrictive
	Steady and flashing violet
	Steady orange		N/A	Most restrictive

Pedestrian signals are for the most part in different colors from motorist signals in order to avoid scenarios like the following. A motorist on a curved roadway is approaching an intersection, and all he can see is a green pedestrian signal because trees block view of his own signal, which is red. The driver mistakes the pedestrian signal to be for vehicles, and consequently does not reduce his speed. After he rounds the curve he discovers that his own light is red, but since he is driving too fast to stop he runs the red signal. Such is a reason that pedestrian signals (at least in the United States) have not used red, yellow or green for many years.

White lane-control signals for motorists have been designed so that they will not likely be confused with white pedestrian signals. Two points of contrast are that the lane-control signals use nothing but arrows and must be hung over the centers of vehicular lanes, while the pedestrian signals instead show the figure of a human being, and are never hung over traffic lanes.

Another helpful distinction is that the violet pedestrian signal is always a triangle, while the violet transit signal is never of that shape.

2B. Clearance for Motorists. Green signals must usually be followed by some kind of clearance indication, because drivers approaching a green light are not preparing to stop. A sudden change to something more restrictive might cause them to brake in panic, which leads to skidding and rear-end collisions. An exception is the progression from flashing green to steady green, which is safe because it only increases privileges for drivers.

Clearance indication is not especially valuable in the progression from flashing red to steady red, because under the flashing red signal all drivers must first stop and then yield before proceeding. Hence panic braking should not occur when the light changes directly to steady red. (It is true that a signal for clearance from flashing red to steady red would be somewhat useful in calling drivers' attention to the progression. However, such a signal was omitted from TFGAP because a solution could not be found that was simple, logical, and compatible with traditional signaling.)

Clearance in lane control warns drivers about lanes, on an intersection approach, from which certain movements will soon be prohibited. Thus informed, drivers intending those movements can select other lanes, and avoid risky last-second lane changes.

2C. Clearance for Pedestrians. This is simpler than clearance for motorists, because panic stops are not an issue. Nonetheless, some indication is required when crossing privileges are soon to end for those on foot. The traditional clearance signal announces that pedestrians should not attempt to begin crossing the street, but pedestrians in progress may comfortably continue. TFGAP's countdown signals add a measure of flexibility because some people walk much faster than others.

The main factor in calculating motorist clearance time at an intersection is the expected speed of the approaching drivers. Altogether different is the calculation of clearance time for pedestrians, which is based largely on the time they need to cross the street, which in turn is primarily a function of street width.

2D. Omission of Future Information. For motorists, a yellow signal gives no hint as to the upcoming signal, except that it will be more restrictive. As an example, a driver facing a steady yellow light has no clue whether he will next see flashing green, flashing red, or steady red. For pedestrians, the flashing white signal portraying a human does not specify whether the next light will be the steady orange hand or the steady violet triangle.

TFGAP omits this information to keep the vocabulary small and to be consistent with the practice, in the United States, of not giving hints about future signals except when travelers must be warned that a privilege will soon end. Not all nations observe this principle, some for instance programming a yellow phase in the sequence from red to green, so drivers with manual transmissions can shift into gear.

2E. Compatibility for Motorists. Certain traditional signals will be retained, even though they are not quite consistent with TFGAP principles. These exceptions are:

1. The steady green and yellow dots are protected for straight movements and permissive for turns. These are permanent exceptions.
2. The steady red dot allows the permissive right turn after stop. This too is a permanent exception.
3. The flashing yellow dot means the same as the steady green dot. This is a temporary, but long-term, exception.
4. The flashing yellow arrow means the same as the steady green arrow. Because educational signs can take care of this matter, this exception is short-term.

Of traffic lights already in service, only those that use flashing yellow will need alteration.

Under traditional rules, a flashing yellow dot at an intersection means the same as a steady green dot. To fully implement TFGAP, these flashing yellow dots would need to be converted to steady green dots. Meanwhile, flashing yellow dots on warning signs would be converted to flashing blue dots. After a period of years, motorists will have forgotten the original meaning of the flashing yellow dot, and then this symbol could be used to show clearance for the flashing green dot. Until then, one method for terminating a flashing green dot is to upgrade it to a steady green dot, followed by the steady yellow dot for clearance. A disadvantage of this workaround is that cross traffic must be stopped in order to make the steady green dot possible.

While the little-used flashing yellow arrow is protected in traditional practice, it is permissive in TFGAP. Since in TFGAP it will be used only with the flashing green arrow, which is also permissive, a YIELD ON FLASHING ARROW sign will handle the incompatibility during the educational period.

On the one hand TFGAP has a signal, the flashing green dot, indicating that all movements are permissive. On the other hand, there is no counterpart signal indicating that all movements are protected, and this absence is a result of the first listed exception. Such a traffic light would be needed only rarely, because most intersections allow pedestrians to cross; but when the all-movement-protected message is essential, it can be effected with a combination of steady green arrows. (As with clearance of flashing red, TFGAP would have included a signal for all-movement protection had a suitable one become apparent.)

2F. Compatibility for Pedestrians. Currently, the signal for clearance of a protected movement is the flashing orange hand, and there is no signal for any kind of permissive movement. TFGAP introduces the steady violet triangle (which resembles a YIELD sign) for the permissive crossing, and further introduces distinct signals for protected and permissive clearance, the flashing white person and flashing violet triangle respectively.

Municipalities will face no urgency in converting older signals. One reason is that the steady white person and steady orange hand will have the same meaning as always. Another reason is that TFGAP never uses the flashing orange hand; thus any pedestrian who encounters that signal may safely assume that it still bears its traditional meaning, that of protected clearance.

2G. Subtleties of Yielding. Toward maintaining simplicity:

1. The flashing green or red signal tells a motorist that he must yield to someone, but it does not specify whom -- other motorists, bicyclists, pedestrians, equestrians, trolleys, etc. Nor does the signal ordinarily indicate from what direction traffic may be coming or in what direction it may be headed, although TFGAP-S offers a first step toward providing such information. Along the same lines, the violet pedestrian signal is equally uninformative. Concerns about complexity mean that travelers who must yield at an intersection will probably never enjoy comprehensive information about signal phasings for all approaches. Instead, road users will always have to take a good look all around before proceeding.

2. TFGAP is not intended to establish a hierarchy of yielding. For instance it cannot effect the following arrangement for motorists at a wye intersection:

• The southwest approach need not yield.
• The southeast approach yields to the southwest approach, but not the north.
• The north approach yields to all traffic from the southeast and southwest.

• A southbound driver going straight need not yield.
• A northbound driver turning left must yield to the southbound driver, but not to a driver on the side road.
• A driver on the side road must yield to both the northbound and southbound drivers.

3-I. Plain Crossings -- Dynamic Service.

As always, if an arrow and dot are both displayed to drivers in the same approach, the arrow overrides the dot for the direction that the arrow indicates.

Table One
Signal	Meaning	Clearance Signal
Steady green dot	Straight movements are protected, turns are permissive	Steady yellow dot
Flashing green dot	All movements are permissive	Flashing yellow dot
Flashing red dot	All movements are permissive after stop	None
Steady red dot	No movement, except permissive right turn after stop	N/A
Steady green arrow	Protected movement in the indicated direction	Steady yellow arrow
Flashing green arrow	Permissive movement in the indicated direction	Flashing yellow arrow
Flashing red arrow	Permissive movement, after stop, in the indicated direction	None
Steady red arrow	No movement in the indicated direction	N/A

3-U. Lane Use.

Lane use signals govern a long stretch of roadway, which may include multiple intersections and driveways. Motorists can permissively turn right from the right lane unless there is a contrary signal.

Table Two
Signal	Meaning	Clearance Signal	Comments
Steady green arrow	Protected movement in the indicated direction	Steady yellow arrow	Used mainly for straight movements
Flashing green arrow	Permissive movement in the indicated direction	Flashing yellow arrow	Used mainly for left turns
Flashing red arrow	Permissive movement, after stop, in the indicated direction	None	Rarely used
Steady red arrow	No movement in the indicated direction	N/A
Steady red X	No use of this lane for any movement	N/A
Flashing blue arrow	Warning of oncoming traffic in same lane	N/A	Used in the two-way left-turn lane.

3-C. Lane Control.

Lane control signals are white arrows displayed over the lanes leading to an intersection, and they regulate from which lanes drivers may turn left, go straight or turn right at that one intersection. The arrows are normally steady but flash during clearance, when the lane configuration is about to change.

3R. Roundabouts.

When a roundabout must be signalized, a key-shaped signal is recommended at its merge points, where all movements are essentially straight. The key reminds drivers that the intersection is indeed a roundabout, and it also solves a clearance signaling problem caused by the delayed availability of the flashing yellow dot.

Table Three
Signal	Meaning	Clearance Signal
Steady green key	Protected movement	Steady yellow key
Flashing green key	Permissive movement	Flashing yellow key
Steady red key	No movement	None
Flashing red key	Permissive movement after stop	N/A

Should upstream metering be needed, it can be handled as in table four, below.

3-M. Metering.

At metering signals, movements are always protected, so lights must always be steady.

Table Four
Signal	Used in	Meaning	Clearance Signal
Steady green dot	4-phase	All drivers may proceed	Steady yellow dot
Steady green numeral 1	2- and 4-phase	One driver may proceed, all others must wait	None
Steady red dot	2- and 4-phase	All drivers must wait	N/A
Steady green hourglass	3-phase	All drivers may proceed	Steady yellow hourglass
Steady red hourglass	3-phase	All drivers must wait	N/A

3-S. Plain Crossings -- Static Service.

The signals here are similar to those of table one. If the nature of the static service never changes, the flashing red, flashing green and steady green dots will suffice. If at some intersection there is periodically a change from one configuration of static service to another, the steady red, flashing yellow and steady yellow dots may also be needed. A few intersections will benefit from arrows.

An optional new symbol is the flashing red numeral, used with or without the flashing red dot to indicate the number of "ways" of the stop. For instance, if the main road of a tee intersection has steady green dots, the side road could be shown a flashing red numeral 1 to indicate that this is a one-way stop; in other words, cross traffic does not stop. Where a minor road fully crosses a major, the major road should have steady green dots, and the minor approaches flashing red 2s. In the case of an all-way stop, we suggest substituting the letter A for the number. These signals correspond to the plaques, attached to stop signs, reading 2-WAY, ALL-WAY et cetera.

A flashing green signal means that cross traffic has steady green. With that established, the flashing green numeral offers so small an increment of information that it is not worth implementing. As a result, numerals of static service will always be flashing red, which contrasts adequately with the steady green numerals of metering signals.

3-B. Beacons.

For emphasis, a flashing blue dot may be attached to a warning sign, barricade, DO NOT ENTER sign, speed limit sign, et cetera. Currently, flashing yellow dots are used for this purpose. The blue dot is of little meaning when used alone, and should always be attached to a descriptive device. The blue dot is not for controlling right-of-way at intersections, although it may be attached to a warning sign whose message is SIGNAL AHEAD. The flashing blue dot is also appropriate for the PREPARE TO STOP WHEN FLASHING sign.

A stop sign beacon remains the traditional flashing red, while the yield sign has flashing green. These lights may be arrows instead of dots if directional information is needed.

How the steady blue signal might be used is yet unknown.

3-P. Pedestrians.

The signal for permissive crossing has little history, but in areas of light traffic pedestrians routinely cross anywhere they please after yielding, so pedestrian permissivity is nothing new.

Table Five
Signal	Meaning	Clearance Signal
Steady white person	Crossing is protected	Flashing white person
Steady violet triangle	Crossing is permissive	Flashing violet triangle
Steady red arrow	Crossing is prohibited	N/A

3-T. Transit.

These signals differ from those in the previous categories because transit signals need not be understood by the general public, but only by transit drivers. Still, when signals of this category fall within view of ordinary travelers, they must exhibit a distinctive appearance so that the public can be readily educated in what devices it needs to ignore. Hence, TFGAP signals for transit operators are always bar-shaped symbols in the color violet.

Within this limitation, bus and rail companies can use any signal to mean anything they choose. Different transit agencies can in fact use different signaling schemes, because each can instruct its vehicle operators in the local policies. (By parallel, signaling conventions for freight railroads have often differed from one company to another.) Still, a national standard in transit signaling can be of benefit: first, it may simplify the manufacture of equipment, reducing costs; and second, a common signaling practice will ease matters when previously separate transit systems are consolidated.

In TFGAP's recommendation, a horizontal double bar means stop and a non-horizontal single bar means to proceed; the vertical or oblique orientation of the single bar specifies in what direction the vehicle will go. Signals are normally lit steadily, but may flash when about to change.

4A. Flashing for Protection. In some parts of the world, the protected left turn is signaled by the flashing green arrow, not the steady. This practice is opposite that in the United States.

These are the signals that would differ in a straightforward adaptation of TFGAP:

• Steady green arrow -- for permissive movement
• Flashing green arrow -- for protected movement
• Steady yellow arrow -- for clearance of steady green arrow
• Flashing yellow arrow -- for clearance of flashing green arrow

Red signals would probably not need to be changed. Meanwhile, the white signals for lane control have little precedent, and the blue signal none, so change is not a question with those.

The exception for the steady green and yellow dots, because they are ubiquitious, would presumably survive: they would continue to indicate that straight movement is protected while turns are permissive. That leaves the flashing green dot as the only remaining symbol. If it is assigned to indicate a permissive move in any direction, an inconsistency arises: the flashing green arrow would be protected but the flashing green dot would be permissive. Then no clear criterion would be evident for whether the numeral and hourglass of metering should be steady or flashing. As for roundabout keys, ought the association be steady-permissive with flashing-protected, or steady-protected with flashing-permissive? No good resolution is conspicuous, and the inconsistency will hamper driver education.

In short, TFGAP cannot readily be adapted to use of the flashing green arrow for protected movement. Not ruled out of course is a broader solution to the problem.

4B. Multiple Lights. A novel method of signaling clearance to mototists is to display a yellow light simultaneously with the green or flashing red signal it is terminating. Specifically, the traffic light shows yellow whenever it is moving to a more restrictive phase; no yellow is needed when restrictivity is decreasing. Hence the steady red signal never calls for clearance, because it is the most restrictive signal possible.

	Normal	Clearance
Least restrictive
Most restrictive		N/A

If a signal has two green sections, it will also need two yellow sections, as the greens may not always be terminated simultaneously. Example:

Advantages of this method:

• It requires no new signal displays.
• It provides a way of signaling clearance for the flashing red when it will be changing to steady red.

• It may require new signal controllers.
• It entails huge lane-use signals.
• All traffic signals will need to be changed.
• Some driver education will be involved.

4C. Flashing for Clearance. The inconsistencies tabulated in section 2A could be resolved by a more thoroughgoing revision of traffic lights, where all non-clearance phases use steady lights, and all clearance phases use flashing lights. TFGAP pedestrian signals already work this way. Such signals might be implemented thus:

Signal	Meaning to Motorists	Meaning to Pedestrians
Steady green	Protected movement
Flashing green	Clearance of protected movement
Steady yellow	Permissive movement
Flashing yellow	Clearance of permissive movement
Steady red	Permissive movement after stop
Flashing red	Clearance of permissive movement after stop
Steady blue	Prohibited movement
Flashing blue	Warning	Warning
Steady white	Lane control	Protected movement
Flashing white	Clearance in lane control	Clearance of protected movement
Steady violet		Permissive movement
Flashing violet		Clearance of permissive movement
Steady orange		Prohibited movement
Flashing orange	not used

Unlikely is conversion from the present system to that given in the table, due to numerous practical problems.

• July 2004. In researching the history of the left-turn arrow, we discovered Kittelson's study. Kittelson was certainly correct in observing that some intersections will be improved with a special signal for permissive turns. Not disclosed was the rationale behind Kittelson's choice of the flashing yellow arrow. In particular, we could not find anywhere in Kittelson's documents an explanation of why the flashing yellow arrow would be superior to the flashing green arrow.
• September 2004. The original TFGAP page, now known as TFGAP-I, was posted.
• November 2004. Lane-use signals (then known as TFGAP-L) were added.
• December 2004. Roundabout signals (TFGAP-R) were added.
• January 2005. Lane-control signals were added, and attached to the lane-use page. Some reworking of the graphical appearance of the lane-use displays became necessary to increase contrast between the two categories.
• February 2005. Metering signals (TFGAP-M) were added as an outgrowth of roundabout signals.
• March 2005. This page (TFGAP-A) and static service (TFGAP-S) were added.
• April 2005.
  • Beacons (TFGAP-B) were added.
  • Many images were improved, with a reduction in both jpeg artifacts and file size.
  • TFGAP-P arrived. Other pages were altered to reflect the importance of pedestrians.
  • Lane use (now called TFGAP-U) and lane control (now TFGAP-C) were separated. Yet again were the lane-use signals reworked.
• May 2005. Transit signals (TFGAP-T) were posted.
• October 2005. The site was moved from home.earthlink.net/~tamivox/dave/traffic to home.comcast.net/~tamivox/dave/traffic.

6A. Site Map.

Branch	Main page	Other pages
TFGAP-A	About TFGAP
TFGAP-I	Plain crossings
TFGAP-U	Lane use
TFGAP-C	Lane control
TFGAP-R	Roundabouts
TFGAP-M	Metering
TFGAP-S	Static service	Examples 4A 4B 4C
TFGAP-B	Beacons	Examples 2A 2B 2C 3A 3B 3C
TFGAP-P	Pedestrians
TFGAP-T	Transit

6B. Growth. We originally had no hint that TFGAP might ever extend beyond for the flashing green arrow for permissive turns at intersections, and a somewhat irregular structure of directories and links has evolved as extensions were linked via the path of least resistance. TFGAP-I had functioned as the main page, because it was the earliest page and because it contains principles that appear in many other pages. However, the first three sections of this page provide a broader introduction.

Additionally, the name TFGAP ("This Flashing Green Arrow Plan") is receding from pertinence, having nothing to do with metering for instance. As pages have been added over the months, the articulation of clearance intervals has greatly risen in prominence, which subject may at some point provide the basis for a general reorganization.

6C. Implementation. The TFGAP site uses mainly text with minimal graphics. A fancier presentation is of course possible, but would probably be slower to download and more likely to malfunction. Moreover, search engines such as Google are most effective with pages that are static rather than dynamic, and pages that contain text rather than images.

Opinion: There is also an issue of environmentalism here. With the internet there is currently a vicious circle:

• Pages become larger, so bandwidth has to be increased.
• Once bandwidth is increased, designers take advantage of that fact to create even larger pages.

6D. Acknowledgements.

Sunflower for tolerating. Elizabeth for proofreading. David Chesler for prompting analysis. Ron Newman for prompting beacons. Mike Moroney for prompting animation patterns. Jon Enslin for prompting red numerals. Editor Marc Fannin of dmoz, for including a link to this site. Many of the folks at Usenet misc.transport.road for their reactions.

Cockatiels Bridget and Bonzer for providing entertainment.