INTRODUCTION TO TRAFFIC SIGNALS This Traffic Signal Design course provides an introduction to intersection control, with an emphasis on the use of traffic signals. Also included in this course is a discussion of MUTCD requirements and safety concerns relative to intersection control and traffic signal operation. The primary national reference document with respect to traffic signals is the Manual on Uniform Traffic Control Devices (commonly referred to as the "MUTCD"), which is published by the Federal Highway Administration. The first MUTCD was published in 1935 with updated manuals being produced on an intermittent basis at roughly ten year intervals. The most recent MUTCD has 10 "parts" (chapters) and was published in 2003. PART 1: GENERAL Part 1 of the MUTCD discusses the purpose of traffic control devices, outlines the responsibility for traffic control devices, and describes how the manual is updated. Definitions and standard abbreviations are also provided for words and phrases used in the manual. www.suncam.com Copyright 2010 Jeffrey W. Buckholz Page 1 of 24
The MUTCD states that the purpose of traffic control devices, including traffic signals, is to: "...promote highway safety and efficiency by providing for the orderly movement of all road users on streets and highways..." The key to ensuring "orderly and predictable movement" is the use of uniform traffic control devices, that is, standard devices that treat similar situations in the same way. For this reason, the use of unique or custom made devices is generally considered undesirable. It is important to understand that uniformity can also be lost if a standard device is used in a nonstandard location, such as would be the case if a traffic signal were inappropriately installed on a freeway mainline. Design, placement, operation, and maintenance of traffic control devices are the responsibilities of the agency having official jurisdiction over the area in question. In most cases this is the state, county, or city with federal agencies having little direct involvement. The major exception is on military bases where federal agencies do play a direct role. Responsibilities with respect to traffic control devices are typically spelled out in legislation and intergovernmental agreements; and it is common for areas of responsibility to be allocated amongst more than one agency. For example, state law might require that the state design and construct all traffic signals located on state highways with the local city or county being responsible for operation and maintenance of these signals. The MUTCD allows states to develop their own manual on traffic control devices. However, the MUTCD requires that the state manual be "in substantial conformance" to the federal MUTCD. The MUTCD makes it clear that, although the manual provides standards for the application and design of traffic control devices, it is not a substitute for sound engineering judgment. Qualified engineers are still needed to properly locate and design traffic control devices. This is especially true in the case of traffic signals where placement, design and timing involve relatively complex issues. The MUTCD uses the verbs "Shall", "Should", and "May" to denote the requirement for varying degrees of conformity with its directives. "Shall" is the most restrictive term, indicating a mandatory condition that must be met. Such mandatory conditions are referred to as Standards. An example standard is: A yellow signal indication shall be displayed following every circular green or green arrow signal indication. "May" is the least restrictive term, indicating a permissive condition that leaves the choice to the designer. Such permissive conditions are referred to as Options. An example option is: Pushbutton locator tones may be used with accessible pedestrian signals. It is left up to the discretion of the designer as to whether or not to use locator tones. www.suncam.com Copyright 2010 Jeffrey W. Buckholz Page 2 of 24
"Should" is an advisory condition which recommends that a certain action be taken, but does not require it. Such advisory conditions are considered Guidance. An example of such guidance is: Traffic control signals within 1/2 mile of one another along a major route or in a network of intersecting major routes should be coordinated. The indication is that this is a highly desirable thing to do; however, for one of many valid reasons, the designer may decide that it is best not to coordinate a given pair of signals. The MUTCD permits any person or organization to submit to the Federal Highway Administration (FHWA) a request for a change. This change might involve modification of an existing device, the use of a new device, or changes to the way in which a device is applied. Although many changes have been granted by the FHWA, in most past cases the requested change has been considered unnecessary and has been rejected to preserve uniformity. If any agency desires to experiment with a new traffic control device, such as field deploying a new device for the purpose of testing it, they must file a request with the FHWA. The FHWA may or may not permit the requested experimentation. The FHWA also provides written interpretations as to the exact meaning of requirements contained within the MUTCD. PART 2: SIGNS Part 2 of the MUTCD discusses signing standards and is divided into ten sections: A. General Standards B. Regulatory Signs C. Warning Signs D. Guide Signs on Conventional Roads E. Guide Signs on Freeways and Expressways F. Specific Service Signs G. Tourist Oriented Directional Signs H. Recreational and Cultural Interest Area Signs I. Emergency Management Signing Unique colors and shapes are used to differentiate the various types of road signing. Signs (such as SIGNAL AHEAD or NO TURN ON RED ) are often an integral part of a traffic signal installation and guidance is provided in this part of the MUTCD on the proper application of these signs. www.suncam.com Copyright 2010 Jeffrey W. Buckholz Page 3 of 24
PART 3: MARKINGS Part 3 of the MUTCD discusses pavement and curb marking standards. Included in this section are pavement marking standards for crosswalks, stop lines, and pavement arrows at signalized intersections. Sections are also provided that deal with the use of object markers, delineators, colored pavements, channelizing devices, and islands. PART 4: TRAFFIC SIGNALS Part 4 of the MUTCD discusses the operation of traffic signals and the eight traffic signal warrants that are used to evaluate the need for a traffic signal installation. Sections are also provided dealing with pedestrian signals, emergence vehicle signals, signals at freeway entrance ramps, signals at moveable bridges, signals on one-lane roads, and lane use control signals. In addition, information is provided on the use of flashing beacons and in-roadway lights. PART 5: LOW-VOLUME ROADS Part 5 of the MUTCD discusses traffic control devices for Low-Volume Roads. A low volume road is defined as a facility lying outside of the built-up areas of cities, towns and communities and having a traffic volume of less than 400 vehicles per day. PART 6: TEMPORARY TRAFFIC CONTROL Part 6 of the MUTCD discusses measures for safely controlling traffic during construction, maintenance or utility work. Sections are included on pedestrian and worker safety, flagging, work zone signing, work zone barricades and pavement markings, lighting of the work zone for nighttime work, detours, and lane closures. A section is also provided which deals with the control of traffic through areas where a traffic incident (such as an accident or natural disaster) has occurred. PART 7: SCHOOL AREAS Part 7 of the MUTCD discusses traffic control measures for school zones, including special school zone signing and pavement marking. Sections are provided on the use of pedestrian crossing guards in school areas and the installation of grade-separated pedestrian crossings, such as a pedestrian overpass. The design and operation of traffic signals in school zones is also discussed. www.suncam.com Copyright 2010 Jeffrey W. Buckholz Page 4 of 24
PART 8: HIGHWAY-RAIL GRADE CROSSINGS Part 8 of the MUTCD discusses traffic control measures for highway-rail grade crossings, including railroad signs and markings and the operation of railroad crossing signals and gates. Requirements for traffic signal preemption at highway-rail grade crossings are also discussed. PART 9: BICYCLE FACILITIES Part 9 of the MUTCD discusses traffic control measures for bicycle facilities, including signs and pavement markings for bikes. Accommodating bicycles at signalized intersections is also addressed. PART 10: HIGHWAY-LIGHT RAIL TRANSIT GRADE CROSSINGS Part 10 of the MUTCD discusses traffic control measures for highway-light rail transit grade crossings, including signs, illumination and markings and the operation of light-rail transit grade crossing systems. Requirements for traffic signal preemption at highway-light rail transit grade crossings is also discussed. The FHWA also publishes the Traffic Control Devices Handbook. This handbook expands upon the material contained in the MUTCD and provides guidelines for implementing the standards contained in the MUTCD. The latest edition of this handbook was published in 2001. In designing, installing, and operating a traffic signal control system there are four basic goals that we want the system to meet. We want the system to be: 1. SAFE 2. EFFICIENT 3. FAIR 4. AFFORDABLE Of highest priority is to make the signal system safe for all users. Potentially dangerous situations should be avoided to the maximum extent possible. To make a traffic signal system efficient we usually want to minimize the number of stops a driver is required to make and the delay associated with those stops. A side benefit of the implementation of an efficient system is low fuel consumption and reduced vehicle emissions. www.suncam.com Copyright 2010 Jeffrey W. Buckholz Page 5 of 24
For a fair system to exist, one group cannot be unduly penalized for the benefit of another. For example, motor vehicle travel could be enhanced by completely eliminating pedestrian crossings. However, this would not be fair to all users of our roadway system. Probably the most difficult goal is the development of a system that is affordable. Almost anything can be accomplished if enough money is allocated to it. The trick is to have a safe, efficient, and fair system that can be constructed and operated at a reasonable price. Having a transportation system where all major roadways are connected by full interchanges and no one is forced to stop at a traffic signal might be operationally desirable, but it would be extremely costly. Two roads can cross each other in one of three basic ways: One of the roads can be built over the top of the other (an overpass), One of the roads can be built over the top of the other with the two roads connected via a series of on and off ramps (an interchange), or The two roads can intersect at-grade (an intersection) If the two roadways cross each other "at-grade" (they form an intersection) then some form of intersection control is required so that it is clear as to which driver has the right-of-way. There are six basic types of intersection control: 1. No Control 2. Two-Way Stop Control 3. Yield Control 4. Multi-Way Stop Control 5. Roundabout Operation 6. Traffic Signal Control At low volume intersections with adequate sight distance it is permissible to have no formal intersection control. In this case, drivers must revert to the "rules of the road" which state that the first driver to reach the intersection has the right-of-way and, if the two drivers arrive at the intersection at about the same time, the driver on the right has the right-of-way. When application of the standard right-of-way rule is unduly hazardous, such as when the major street is busy or a sight distance restriction exists, the use of two-way stop control may be www.suncam.com Copyright 2010 Jeffrey W. Buckholz Page 6 of 24
appropriate. STOP signs are typically installed on the minor street, which is the street having either the lower traffic volume or the lower speed (or both). Where adequate sight distance exists, intersection control using yield signs instead of stop signs may be appropriate. As with stop signs, yield signs are typically installed on the minor street. When the traffic volume on the intersecting roads is about equal, multi-way stop control or the use of roundabouts becomes an attractive option. With multi-way stop control all motorists entering the intersection must come to a complete stop before making their desired movement, doing so in a first-come-first-served fashion. Under roundabout control, a round central island is installed in the middle of the intersection and a circulating roadway is constructed around the island. All entering traffic must yield to traffic already on the circulating roadway, entering only when a suitable gap in the traffic stream exists. A true roundabout differs from a traffic circle or a rotary in that it has certain design features which enhance its operation. For example, a true roundabout incorporates raised triangular splitter islands where the approaching street intersects the circulating roadway. These splitter islands encourage approaching motorists to reduce their speed and ensure that the path of approaching vehicles deflects to the right of the central island. It should be noted that there are numerical warrants contained within the MUTCD for multi-way stop control and these warrants should be met before multi-way stop control is installed. There are no such numerical warrants for roundabouts in the MUTCD. www.suncam.com Copyright 2010 Jeffrey W. Buckholz Page 7 of 24
Stop and yield signs are used when traffic volume on the minor street is relatively low. When minor street traffic volumes are high, then a more sophisticated form of intersection control is needed, such as a traffic signal. At a traffic signal, the right-of-way is assigned through a relatively complex series of red, yellow, and green "arrow" and "ball" indications, each with its own meaning. In addition to assigning the right-of-way amongst vehicular traffic, modern traffic signals also provide right-of-way control for pedestrians so that they may safely cross the street; and some traffic signals are even designed to accommodate light rail transit vehicles. If designed and installed properly, traffic signals have many advantages with respect to other forms of intersection control. Since traffic signals clearly inform each driver of the right-of-way situation, little driver judgment is needed. Simply put, the driver need only go when the signal is green and stop when it is red. Other forms of traffic control (stop signs and roundabouts) require the driver to make more complex decisions, such as gauging the availability of suitable gaps in the opposing traffic stream. Reducing the amount of driver judgment required is generally considered beneficial since the less judgment a driver is required to exercise the less chance there is that a mistake will be made. Traffic signals are capable of efficiently handling relatively high volumes of traffic, even at complex intersections. This is not the case with stop or yield controlled intersections where vehicle queues and associated vehicular delay can become excessive if traffic volumes at the unsignalized intersection grow large. The assignment of right-of-way at an intersection is more "fair" with a traffic signal than with other forms of intersection control. At stop signs or roundabouts, vehicles desiring to enter the main traffic stream must wait for a suitable gap. If under high volume conditions it takes an inordinate amount of time to find such a gap then the entering driver simply has to wait. However, with a traffic signal the driver is assured that he or she will eventually be "given their turn" to proceed thru the intersection. The time that the entering driver has to wait is also controllable with traffic signals whereas it is not with stop signs or roundabouts. One need only change the timings in the traffic signal controller to change the amount of time that the entering driver is required to wait. Modern signal controllers also allow this waiting time to vary by time-of-day and day-of-week. The installation of a traffic signal generally reduces the number of right-angle accidents and, if left turn phasing is provided, the number of left turn accidents. If the intersection is properly designed and pedestrian signals are provided, it is generally safer for pedestrians to cross busy streets at a signalized intersection than at an unsignalized one. When a non-correctable sight distance www.suncam.com Copyright 2010 Jeffrey W. Buckholz Page 8 of 24
restriction exists, such as when a building is located too close to the corner of the intersection, a traffic signal may be the only safe method for assigning intersection right-of-way. SIGHT DISTANCE Although traffic signals offer many advantages, they also have certain disadvantages with respect to other forms of intersection control. The installation cost of a traffic signal is much greater than for stop sign control and, depending on the situation, may also be greater than the cost of installing a roundabout. A combined design, construction, and inspection cost of from $70,000 to over $200,000 is typical for modern traffic signal installations. This is much more expensive than a STOP sign installation, which usually costs less than $5000. Once a traffic signal is installed, money must also be spent to power the traffic signal and to maintain, repair, and periodically retime the signal. These costs, which are ongoing over the life of the signal installation, can amount to a substantial amount of money. When traffic volumes at an intersection are low the high degree of control offered by traffic signals results in a substantial increase in overall motorists delay in comparison to other forms of intersection control. Stop signs, yield signs, or roundabouts are usually better forms of intersection control under low volume conditions. If situated in the wrong location, new traffic signals can also disrupt coordinated traffic flow along an arterial. Large platoons of traffic using the arterial may be forced to stop at the new signal, which substantially increases system delay and frustrates drivers. When a traffic signal is installed, the number of rear-end accidents on the main street can be expected to increase. This is only logical since, with the installation of a traffic signal, main street traffic is now required to stop where previously it kept going. A counterbalancing factor is that www.suncam.com Copyright 2010 Jeffrey W. Buckholz Page 9 of 24
rear-end accidents (which tend to increase with the installation of a signal) are generally not as severe as right-angle accidents (which tend to decrease with the installation of a signal). A traffic signal installation is also a rather complex apparatus and there are many things that can go wrong, from equipment failure to power outages. If this happens during a busy traffic period the negative effect on both safety and congestion can be substantial. Finally, although traffic signals can be designed with good aesthetics in mind, no matter how nicely you package them they are still rather ugly devices. Stop signs and roundabouts are considerably less obtrusive with respect to the surrounding visual environment. Contrary to what many citizens think, not every traffic problem is best corrected by installing a traffic signal. Sometimes geometric modifications or signing and striping changes are in order. A traffic signal is not a panacea for every traffic-related problem. The bottom line is that traffic signals should only be installed where the expected advantages outweigh the expected disadvantages. An engineering study is required to make the proper decision. In many instances, roundabouts designed with multiple circulating and entry lanes are capable of providing suitable traffic control at intersecting streets carrying substantial traffic volumes. MULTILANE ROUNDABOUT EXAMPLE OR OR Such multilane roundabouts represent a significantly more complex right-of-way assignment situation than single lane roundabouts. Vehicles in the center circulating roadway of a multilane roundabout are generally restricted in the movements that they can make from a given lane (i.e. www.suncam.com Copyright 2010 Jeffrey W. Buckholz Page 10 of 24
where they can exit the roundabout) whereas those in a single lane roundabout are not. The primary advantage of multilane roundabouts, if drivers know how to properly use them, is that they are capable of handling much higher volumes of traffic than single lane roundabouts. The primary disadvantage is that they require a high level of motorist familiarity with roundabout lane discipline; a level of familiarity that motorists in many other countries have mastered but that US motorists are still developing. In comparison to traffic signals, roundabouts have certain advantages and certain disadvantages. Advantages include: Lower motorist delay under moderate volume conditions No power cost and low maintenance & repair costs Not subject to power outages or equipment failures Attractive if properly landscaped Accidents are typically minor in nature, such as sideswipe accidents Disadvantages include: Potentially large right-of-way requirements Lack of driver familiarity in the US, especially with respect to multilane roundabouts Need for a higher level of driver judgment Adverse effect on signal coordination if situated within a progressed arterial Somewhat awkward accommodation of bicycle and pedestrian traffic Potentially "unfair" right-of-way assignment, especially if the approach to one s immediate left is a busy one. To avoid the extensive queuing and delay that can result from this last disadvantage, some roundabouts use traffic signals to assign the right-of-way at high volume entry points into the circulating roadway. However, such a design begins to challenge the definition of a true roundabout since true roundabouts do not utilize signal control. Traffic control at an intersection can also be accomplished through changes to the basic geometry of the intersection. An example of this is the installation of a raised island in the middle of the major street. The resulting directional median opening physically restricts left turns onto the major street or thru movements from one side street to the other. www.suncam.com Copyright 2010 Jeffrey W. Buckholz Page 11 of 24
Although traffic signals are most commonly used to control vehicular movements at the intersection of two roads, they are also used at: The intersection of freeway off-ramps and local streets, Mid-block pedestrian crossings, Mid-block fire stations, Moveable bridges, 1-lane, 2-way work zones, and Freeway on-ramps (for ramp metering) Unless there is a good engineering reason, a traffic signal should not be installed at an intersection unless at least one of the 8 traffic signal "warrants" contained in Part 4 of the MUTCD is met. These 8 warrants are numerical and operational guidelines used to evaluate the need for a traffic signal. However, it should be understood that the satisfaction of a warrant is not in itself justification for the installation of a signal. Other measures, such as the construction of a roundabout or median modifications, may provide a better solution. Intersection controls, such as stop signs or traffic signals, should not be used for speed control. Placing unwarranted stop signs or traffic signals at an intersection in an attempt to reduce speeding breeds motorist disrespect for the entire traffic control system and tempts otherwise law-abiding citizens to break the law by disregarding the unwarranted device. Speed control should be obtained thru other acceptable measures such as traffic calming techniques (road narrowing, speed humps, raiser intersections, etc.) or increased speed enforcement. Traffic signal installations are composed of six major components: 1. Signal supports 2. Vehicular signal heads 3. Pedestrian signal heads 4. Pedestrian detection system 5. Vehicle detection system 6. The controller and cabinet Most traffic signals are either supported by a span wire strung between wooden, steel or concrete support poles, or are supported by a steel mast arm structure. Span wire installations are typically www.suncam.com Copyright 2010 Jeffrey W. Buckholz Page 12 of 24
less expensive than mast arm installations and provide flexibility in signal head placement. However, traffic signal heads supported by mast arms with rigid support brackets are less susceptible to being turned or blown down by high winds. In most cases, mast arm installations are also considered more attractive than span wire installations. A signal head contains one or more signal faces and a signal face contains one or more signal www.suncam.com Copyright 2010 Jeffrey W. Buckholz Page 13 of 24
indications. The MUTCD states that, except in pedestrian signals, each signal face shall have at least three but not more than five red, yellow or green signal indications. NUMBER OF SIGNAL LENSES 3 MINIMUM 5 MAXIMUM The MUTCD requires that signal indications be circular and either 8 inches or 12 inches in diameter. To maximize visibility, most modern traffic signal installations use 12 inch diameter indications. Signal indications consisting entirely of colored LED's (Light Emitting Diodes), instead of incandescent bulbs and lenses, are rapidly taking over as the preferred signal display technology. Although there initial cost is somewhat higher, these LED lamps have much lower power www.suncam.com Copyright 2010 Jeffrey W. Buckholz Page 14 of 24
consumption than conventional bulbs and also have a much longer life. LED's are available in all of the required vehicular and pedestrian signal colors (red, yellow, green, white, and orange). A signal head with 4 faces is commonly referred to as a 4-way signal; a signal head with 3 faces is referred to as a 3-way signal, and so on. Many signal heads are made from aluminum, although polycarbonate signal heads ("plastic" heads) are also commonplace. The overall durability of aluminum heads is generally better than that of polycarbonate heads and aluminum heads have a lower susceptibility to deterioration caused by sun-induced ultraviolet (UV) radiation. However, polycarbonate heads weigh less than aluminum heads, are less susceptible to corrosion from ocean air (which is a problem for some seaside communities), and are usually less expensive than aluminum heads. For thru traffic, the MUTCD states that a minimum of 2 signal faces shall be provided. This requirement is made for obvious safety reasons since, if there were only one signal face and the red indication of this face burned out, approaching motorists would see no red indication at all. At signalized "T" intersections, the predominant turning movement on the stem of the "T" must also have at least 2 signal faces: SIGNAL FACES AT "T" INTERSECTIONS The MUTCD states that the 2 signal faces controlling a thru lane shall be horizontally separated by a minimum distance of 8 feet: www.suncam.com Copyright 2010 Jeffrey W. Buckholz Page 15 of 24
HORIZONTAL SEPARATION BETWEEN HEADS Providing sufficient horizontal separation reduces the chance that a large vehicle will simultaneously obstruct the view of both signal faces. For turn lanes, only 1 signal face is required by the MUTCD. Only 1 signal face is required since, if the red indication goes out in this face, motorists still have the adjacent thru lane signal faces to provide some indication as to right-of-way status. If there are dual turn lanes (either dual rights or dual lefts) then the MUTCD states that two signal faces should be provided. For maximum clarity, it is usually desirable to have a signal head aligned with the lane that it controls. However, this is by no means a requirement. The MUTCD also states that signal faces should be visible from the distance indicated in the following table: Note that the required minimum sight distance depends on the 85th percentile speed of the www.suncam.com Copyright 2010 Jeffrey W. Buckholz Page 16 of 24
approaching traffic with higher speed traffic requiring more visibility distance. The minimum visibility table uses the 85th percentile speed, not the average speed or even the posted speed. The 85th percentile speed, which is the speed exceeded by only 15% of the drivers, is typically determined through a traffic engineering speed study. In practice, engineers frequently use the posted speed as an approximation for the 85th percentile speed (since speed limits are typically set using the 85th percentile speed). However, there is no guarantee that the posted speed and the 85th percentile speed are the same. For this reason, a traffic engineering speed study is the most prudent method for determining the 85th percentile speed. When the minimum sight distance cannot be met on an intersection approach, then an advanced warning sign (such as the symbolic SIGNAL AHEAD sign) is required by the MUTCD. If so desired, this sign may be supplemented by a circular yellow Hazard Identification Beacon. When intersections are very closely spaced or when roads intersect at an angle, driver confusion can result if the signal faces for one approach are not shielded from the view of motorists on other approaches: www.suncam.com Copyright 2010 Jeffrey W. Buckholz Page 17 of 24
UNSHIELDED SIGNAL FACES This shielding can be accomplished thru the use of long tunnel visors, louvers, or optically programmed signal heads. Optically programmed signal heads are special heads which are designed to present a narrow field of view to the motorist so that no ambiguity occurs as to which movement the head controls. A backplate is a thin strip of material (usually aluminum or plastic) that extends outward from behind the signal face on all sides of the signal housing. Unlike those shown here, the MUTCD requires that the front surface of backplates have a dull black finish to minimize light reflection. Some agencies use backplates to enhance the target value of the signal face when viewed against a bright sky, or when viewed against potentially confusing www.suncam.com Copyright 2010 Jeffrey W. Buckholz Page 18 of 24
background lighting. The primary disadvantage of backplates is that they have a high wind loading which may require the use of larger signal support poles. The MUTCD requires pedestrian signal heads to be rectangular in shape and consist of two indications: a white walking person, and an orange upraised hand As with signal heads, LED indications are quickly taking over as the preferred means of illumination for pedestrian indications. The MUTCD states that pedestrian signal heads shall be mounted at least 7 feet, but no more than 10 feet, above the sidewalk. Pedestrian signal heads shall be positioned for maximum visibility at the beginning of the controlled crossing and the pedestrian indications should be visible from the beginning of the crossing to a point 10 feet or less from the end of the crossing. RECOMMENDED PEDESTRIAN VISIBILITY AREA The MUTCD requires pedestrian signal indications when one or more of the following conditions exist: www.suncam.com Copyright 2010 Jeffrey W. Buckholz Page 19 of 24
A signal is installed under Warrant #4 (the Pedestrian Volume warrant) or Warrant #5 (the School Crossing warrant). An exclusive pedestrian phase is provided. (With an exclusive pedestrian phase, all vehicle movements are stopped to allow pedestrians to cross.) At established school crossings. The vehicular signal indications would tend to confuse pedestrians. The MUTCD states that pedestrian signal indications may be pertinent when one or more of the following conditions exist: Help is needed for pedestrians to safely cross the street. Pedestrians are required to cross a street in two movements, taking refuge on an intermediate island. The vehicular signal indications are not visible to pedestrians, such as on one-way streets or at "T" intersections. The most common type of pedestrian detector is a simple push button; which works much like an elevator button. When a pedestrian button is pushed a call for the pedestrian phase is "locked" into the traffic signal controller. Then, at the appropriate time, the WALKING PERSON (symbolic WALK) indication, followed by the flashing UPRAISED HAND (symbolic DON'T WALK) indication, are displayed. The WALK indication means that the pedestrian may enter the crosswalk in the direction of the indication. The flashing DON T WALK indication (which represents the pedestrian clearance interval) means that the pedestrian shall not begin crossing the roadway but, if he or she has already entered the crosswalk, the crossing may be completed. The steady DON T WALK indication means that the pedestrian crossing interval has ended and the green indication is about to be given to conflicting traffic. The length of the pedestrian clearance interval in seconds is based on a walking speed of 3.5 feet per second and is typically obtained by dividing the crossing distance in feet by this speed. An extremely popular extension of the typical pedestrian head is the use of countdown pedestrian signals. Countdown pedestrian signals include orange numbers next to the upraised orange hand that indicate the amount of time left in the pedestrian clearance interval. www.suncam.com Copyright 2010 Jeffrey W. Buckholz Page 20 of 24
Although it is considered good engineering practice, the MUTCD does not require the use of marked crosswalks when pedestrian signals are used. Although signals may be placed in flashing operation when not needed, the MUTCD does not permit signals to be turned-off. When a traffic signal is placed into flashing operation, the MUTCD requires that the pedestrian heads be "dark" (that is, have no indication). If there are no pedestrian heads at a signalized intersection then pedestrians are required to cross with the appropriate green vehicular signal indication. The MUTCD states that a permanent sign explaining their purpose and use shall be installed near pedestrian buttons: When two crosswalks oriented in different directions end at or near the same location, the positioning of the pedestrian buttons and the associated signing should clearly indicate the button that pertains to each crossing. The MUTCD recommends a pedestrian button mounting height of approximately 3.5 feet from the sidewalk. This height is convenient for all users, including children www.suncam.com Copyright 2010 Jeffrey W. Buckholz Page 21 of 24
and those in wheelchairs. In addition, the pedestrian button should be conveniently located near the end of the pedestrian crosswalk. Sidewalk access to the pedestrian button is highly desirable so that individuals in wheelchairs can reach it. A relatively new pedestrian feature that is finding increased use is a pilot light on pedestrian buttons. This light is activated when the button is pushed by a pedestrian and is extinguished when the pedestrian receives the WALK indication. The light gives pedestrians the assurance that their request for the WALK indication has been recognized by the traffic signal. The MUTCD permits the installation of mid-block school crossing signals where conditions warrant their use. Mid-block school crossings are free from the hazards of turning vehicles, and may offer added convenience to students. However, they can surprise drivers who do not expect pedestrian crossings or signal control between intersections. Vehicular and pedestrian signal heads operate on 120 VAC current, the same type of current that is used for standard electrical outlets in a residential home. As with a home, this power is supplied to the intersection by the local electric company. The 120 VAC current passes through the traffic signal controller cabinet on its way to the vehicular and pedestrian signal heads. If, due to severe weather or some other unusual circumstance, a power outage occurs at a traffic signal, the traffic signal indications will go dark. Not having any indications at a traffic signal can be very dangerous, especially at major intersections during periods of high traffic volume. To prevent this from happening, many jurisdictions are adding battery-powered uninterruptable power supply units to their traffic signal cabinets: www.suncam.com Copyright 2010 Jeffrey W. Buckholz Page 22 of 24
These units provide supplemental power to the traffic signal when power from the local electric company is absent. If the power outage is for an extended period of time, as might be the case after a hurricane or other severe storm has toppled electric lines, the battery reserve may eventually run dry. In these cases additional supplemental power can be provide by an external generator. Many jurisdictions have pre-installed the required electrical connections at the control cabinet in order to allow such generators to be quickly attached to the traffic signal installation. External sensors located either overhead or in the pavement are used to detect the presence of vehicles on the various approaches to a signalized intersection. This information is then communicated to the controller via 24 volt DC logic circuits. Pedestrian detection provided by pedestrian push buttons is also communicated to the controller via 24 volt DC logic circuits. The "brain" of the traffic signal is the traffic signal controller, which is housed inside a water-tight www.suncam.com Copyright 2010 Jeffrey W. Buckholz Page 23 of 24
cabinet located at the intersection: Using information programmed into it by the traffic engineer or signal technician, as well as external information it receives from the vehicular and pedestrian detection systems, the controller decides who will be given the right-of-way at a particular point in time. The traffic signal controller also supervises the way in which power is supplied to the field equipment. At every moment in time the controller decides which indications will be illuminated. Although the signal controller is powered by 120 volt AC current, decisions are made and communicated to other signal equipment using 24 volt DC logic circuits. All of the major components of the signal system, including the support poles, the controller cabinet, and the electric power service assembly, are grounded using copper ground rods and connecting wire. The grounding system provides protection from unwanted voltage transients, such as those caused by lightning strikes. It does so by providing an attractive electrical path to the earth for these transients to follow, a path that does not pass thru any sensitive (and expensive) electrical equipment. GROUNDING SYSTEM www.suncam.com Copyright 2010 Jeffrey W. Buckholz Page 24 of 24