1A. General. This report is a look at some signals in use or proposed for transit vehicles, which can be defined as passenger vehicles of public transportation, primarily urban, typically able to carry dozens of people. Most transit involves either buses, or rail vehicles that operate on tracks resembling those of freight railroads.

In contrast to transit is familiar ordinary traffic, consisting of the usual motorists, bicyclists or pedestrians, who are driving, pedaling or walking on conventional roads, bike paths and sidewalks; and they observe standard traffic signs, signals, and pavement markings. Except in a few areas that have comprehensive public transportation, the vast majority of travelers are ordinary.

Some transit vehicles operate on an exclusive guideway; almost all subway and aerial lines are so configured, and monorails because of their design virtually require an exclusive guideway. Because the transit vehicles never share the road with ordinary traffic, there is no contention for right-of-way and all travelers should as a result enjoy speedier trips. These systems usually use signals very similar to ordinary traffic lights, employing red, yellow and green dots. Due to their physical separation from conventional roads, the transit signals should not confuse ordinary motorists.

Exclusive guideways often involve high construction costs, so many transit agencies instead operate vehicles on ordinary roads, sharing the right of way. The disadvantage here is that transit vehicles risk delay from the congestion of ordinary traffic, but in mitigation special traffic signals can be installed offering transit vehicles priority of some kind. In some systems, the guideways are mostly exclusive, the only interaction with ordinary traffic occuring at crossings that are roughly perpendicular.

While many buses operate as ordinary traffic, in some cities buses enjoy exclusive lanes or special traffic signals, either accomodation intended to help the bus travel faster. The hope is to make public transportation more appealing to potential passengers, and thus to reduce automotive traffic (and congestion) by increasing bus ridership.

Some rail transit vehicles share extensive roadway with ordinary traffic. In the early twentieth century, they were often called trolleys or streetcars; today they are more likely known as light rail vehicles. The tracks are laid in the street. Ordinary travelers frequently cross the line, and in some systems motorists might drive on top of the tracks, paralleling the rails for a significant distance when no train is present. These motorists enjoy a reasonably smooth ride because the concrete or asphalt pavement is installed very close to the rails, reducing the inevitable gaps to a few centimeters. On the other hand, bicyclists risk difficulty in such parallel travel because their wheels might be small enough to slip into the gaps, especially the larger gaps of switches. Travelers in wheelchairs are usually crossing the tracks, and by their perpendicular motion have somewhat less chance of a wheel getting stuck in the tracks.

1B. Sources. The Transit Cooperative Research Program's Report 17, Integration of Light Rail Transit into City Streets, is an essential source of recent information about rail transit in non-exclusive environments. Also, it contains an important signal proposal discussed later in the present report.

Another valuable document is the Manual on Uniform Traffic Control Devices (MUTCD), particularly its part 10.

A detailed critique of MUTCD part 10 has been prepared by Jack Boorse.

1C. Right-of-Way Classifications. MUTCD section 10A.01 gives some useful transit definitions:

"Light rail alignments can be grouped into one of the following three types:

1. Exclusive: A light rail transit right-of-way that is grade-separated or protected by a fence or traffic barrier. Motor vehicles, pedestrians, and bicycles are prohibited within the right-of-way ...
2. Semiexclusive: A light rail transit alignment that is in a separate right-of-way or along a street or railroad right-of-way where motor vehicles, pedestrians, and bicycles have limited access and cross at designated locations only.
3. Mixed-Use: An alignment where light rail transit operates in mixed traffic with all types of road users. This includes streets, transit malls, and pedestrian malls where the right-of-way is shared."

Although the MUTCD speaks of light rail transit, similar principles apply to bus transit, and the present reports treats them both. In many type C locales, bus drivers observe all the same signs, signals and markings as ordinary traffic. In other areas, special traffic lights include a bus-only PROCEED phase, to expedite passage of these transit vehicles.

In transit of type C, ordinary travelers will certainly be able to see any special signals for transit operators. The same is often true with type B, for instance when the transit line is in the median of a city street and intersections entail grade crossings. Transit signals must be distinct from ordinary traffic lights, so that transit operators and ordinary travelers respectively know which signals to observe and which to ignore. The present report concentrates on the physical appearance of signals for transit operators, while other components of TFGAP address ordinary travelers.

1D. Differences of Bus and Rail. Practically all signaling systems include a symbol for PREPARE TO STOP, but the effect depends of the vehicle technology.

• With block-controlled rail lines, PREPARE TO STOP likely indicates that the operator may proceed past this signal, but will need to stop when he reaches the next signal along the line. Because rail vehicles are often heavy, they can have long stopping distances, and operators need advance notice.
• In bus transit, the PREPARE TO STOP signal more likely means the same thing as a yellow light as an intersection: stop if possible, otherwise proceed.

Similarly, the meaning of the PROCEED LEFT or RIGHT messages depends on vehicle type.

• In rail transit, LEFT or RIGHT is merely advisory, as the train will go wherever the switch setting guides it.
• In bus transit the driver must take action, turning the steering wheel the proper direction.

2A. A Brief Survey. Chapter two of Report 17 cites a number of transit signals in current use, including:

• A vertical white bar directing operators to PROCEED STRAIGHT
• A slanted white bar indicating PREPARE TO STOP
• A pair of horizontal white bars indicating STOP
• Red, yellow and green dots with a TROLLEY SIGNAL sign; the dots have the obvious meaning
• Red yellow and white T symbols, with special light rail vehicle sign; the white T means PROCEED
• Red and green X signals, meaning STOP and PROCEED respectively
• A yellow horizontal bar meaning STOP

The important conclusion here is that transit signals are far from uniform. This greatly complicates driver education, as ordinary travelers visiting an unfamiliar locality do not know what signals they should be ignoring. This lack of consistency is a key motivation behind the proposal of the next section.

2B. The TCRP Proposal. Section 3.7.3 of Report 17 prescribes a system of signals for transit operators. It establishes a first step toward nationwide uniformity by making all signals white, but it does give planners a choice of either three-section or two-section devices for stop-and-go signals. Much of the TCRP plan has been incorporated in MUTCD section 10D.07. Signals from those documents as well as Boorse's, when reproduced in the present report, appear on a shaded background.

Flashing signals are pictured with emanating rays, for example:

Steady	Flashing

2C. TCRP Three-Section Stop-and-Go Signals. According to the TCRP these are "recommended" over the two-section version, but the MUTCD does not express a preference.

Prepare to stop	Stop	Remain stopped, but prepare to proceed
Proceed left	Proceed straight	Proceed right

Because PREPARE TO STOP does not mean STOP, a transit operator may still need directional information. The sources are vague on how to combine directional information with the prepare-to-stop message, but one approach is to show two sections simultaneously:

Proceed left but prepare to stop	Proceed straight but prepare to stop	Proceed right but prepare to stop

Another idea is to rotate the triangle appropriately:

Proceed left but prepare to stop	Proceed straight but prepare to stop	Proceed right but prepare to stop

By its symmetry, an equilateral triangle points in three directions without distinction. Here are examples of images less ambiguous:

Boorse additionally depicts:

• Signals where PROCEED is at the top and STOP is at the bottom; this accords with much traditional rail practice.
• A small circle, below left, as an alternative to the triangle for the prepare-to-stop message.
• T-shaped signals, below right. Naturally, red is for STOP and yellow for PREPARE TO STOP. However, the T-signal for PROCEED is not green but instead white, perhaps because a green T would be misinterpreted as a green arrow by ordinary motorists.

2D. TCRP Two-Section Stop-and-Go Signals. These give no less information than their three-section counterparts.

	Stop		Remain stopped, but prepare to proceed
Proceed left		Proceed straight		Proceed right
Proceed left but prepare to stop		Proceed straight but prepare to stop		Proceed right but prepare to stop

Patently, these could be condensed into a one-section signal:

2E. TCRP Switch-Position Signals. Mentioned in in the TCRP report, they are omitted from the MUTCD. Each arrow points in the direction a switch is set, while the bar if included indicates a higher speed limit. The TCRP indicates that these special signals may be needed when there is a lack of interlocking between stop-and-go signals and switches.

The TCRP switch signals above resemble the intersection arrows below, and might confuse any ordinary motorists who happen to see them:

Boorse mentions the following switch signal. Using a more obtuse angle decreases the similarity to the intersection arrow, but increases the risk that the symbol will be mistaken by the transit operator for a straight bar.

3A. Introduction. TFGAP transit signals are based largely on the one-section signals above, which are themselves derived from the TCRP report. Some changes are in order, however.

Because many sources of general illumination are white, signals of that color are not as conspicuous as might be desired. As a result, TFGAP calls for transit signals to instead be violet in color. Violet is unprecedented in signaling except in TFGAP-P, where it is used for a triangular symbol (below) which will contrast amply with the bar shapes of transit signaling.

The TCRP PROCEED signals are bars turned in any of several directions. When the bar happens to be precisely horizontal, though, it carries according to TCRP the opposite meaning, STOP. This may lead to operator error, as in this example. Suppose at a certain three-tine fork, the tracks go straight, gently left, and sharply left. Then the pertinent signal aspects would look like this:

Proceed straight	Proceed gently left	Proceed sharply left	Stop (TCRP)

The difference of orientation between the PROCEED SHARPLY LEFT and STOP signals is so small that a transit operator might mistake the latter for the former, and by failing to stop cause a crash. For this reason, TFGAP recommends using the double bar to mean STOP.

3B. Concise Specification.

1. Each signal section is a 200- to 300-millimeter circle or square. Normally, one section is sufficient.
2. All aspects use only violet bar symbols on a black background. A dark signal is also acceptable.
3. All portions of the signal face that are lit are of uniform color and brightness.
4. The bar(s) may be horizontal, vertical or oblique. If more than one, they are parallel.
5. The bar(s) may shine steadily, or they might flash on and off. Further, the signal may display an animation if each instantaneous image accords with clauses 2, 3 and 4.

To give transit agencies flexibility in dealing with local conditions, the concise specification does not actually say what each symbol means, but these are the most likely definitions:

	Stop		Remain stopped, but prepare to proceed
Proceed left		Proceed straight		Proceed right
Proceed left but prepare to stop		Proceed straight but prepare to stop		Proceed right but prepare to stop

The PREPARE TO PROCEED signal can be made more specific if necessary:

Remain stopped, but prepare to proceed left	Remain stopped, but prepare to proceed straight	Remain stopped, but prepare to proceed right

Signals for turning can also be more precise:

Proceed gently right	Proceed sharply right

Multiple bars need not be the same length. The signals below are possibilities to convey switch-position information. Each long bar shows the direction of the switch. When the short bar is displayed to the left of the long bar, it indicates a lower speed limit; to the right, higher. If a more refined speed message needs to be conveyed, then a two-section signal will likely be necessary.

Switch to left, low speed	Switch straight, low speed	Switch to right, low speed
Switch to left, high speed	Switch straight, high speed	Switch to right, high speed

3C. Comments.

1. A conspicuous advantage of this plan is its crisp criterion for signal discrimination: operators of transit vehicles should observe only the violet-bar signals, while ordinary travelers should ignore any violet-bar signal. This will in particular simplify education of drivers, bicyclists and pedestrians.

2a. The recommended signals have only one section, because in general that is enough. Also, public transit is most frequently implemented in crowded places, and a one-section signal will fit into some places that the two- or three-signal device will not.
2b. If a particularly wide vocabulary is needed, the signal might have two sections, as when a switch-position signal is mounted with a stop-and-go signal.
2c. Going further to a three-section device runs the risk that ordinary motorists will try to somehow interpret it as a conventional stop-and-go traffic light, at least if the three sections are arranged in a straight line. The workaround is a nonlinear arrangement: of the three examples below, the final uses a hexagon as an approximation to a circle:

This report does not attempt to define what the various signal combinations of a three-section device might mean, but a transit system that needs to use complicated signals will find a way to educate its operators. Fortunately, there is no need to train the general public.

3. If multiple bars are displayed in one section simultaneously, and the bars are not parallel, then depending on the bars' arrangement ordinary travelers might try to interpret the image as an arrow or an X (below left), which shapes are used by traditional intersection and lane-use signals. On the other hand, if a two-section signal is used and both sections are activated simultaneously (below right), there is no great need for the bars in one to parallel the bars in the other.

4. Complicated flashing patterns and animations are acceptable because ordinary travelers will not have to figure out what they mean, and the transit agency will have the opportunity to train its employees in interpreting them. Cases:

• A simple flashing pattern is 0.5 second on, 0.5 second off.
• More complicated is 0.2 second on, 0.2 second off, 0.2 second on, 0.6 second off.
• A simple animation is an alternation between horizontal and vertical bars, each displayed for 0.5 second.
• More sophisticated is a bar spinning clockwise at 0.5 revolution per second.

5. When transit signals are installed in places in which ordinary travelers cannot see them, this specification becomes irrelevant and transit agencies can use any signals they please, including those that resemble ordinary traffic signals. Practically all train yards exemplify this isolation. In particular, the functioning of yards differs so greatly from that of revenue lines that yards may well benefit from special signals which could be for instance red and green dots.

6. With modern radio communication, in-cab signals and two-way voice communication have become common features in rail vehicles. Such messages can be arbitrarily complicated, including text. Nonetheless, fixed signals along the line represent at minimum a valuable backup control system.