| FREQUENTLY ASKED QUESTIONS |
1. Why does the light take so long to change? |
Signals that are in a coordinated mode are confined to a cycle length, which is governed by the cycle of a nearby major intersection. All signals along an arterial must have a common cycle length in order to achieve progression.
Within that cycle length, a block of time is allocated to each movement. Each movement can appear only at a certain point in the cycle; once that has occurred, the movement cannot appear again until the next cycle. If a movement does not need all of its allocated time, the unused time becomes available to the next movement; this continues until all of the unused time, if any remains, ultimately is inherited by the main street movement where the cycle “zeros” itself out or begins again.
Cycle lengths vary depending on the time of day. During the AM and PM rush hours, signals have their longest cycle lengths because the major roads must accommodate the greatest amount of traffic. In the midday the cycle lengths are slightly shorter and the shortest cycles typically occur at night and on weekends.
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2. Why is every light red? Can't they coordinate them? |
There are many issues that may affect the coordination of traffic signals. It is easy to coordinate signals that are on one-way streets to provide for very efficient movement of traffic. That is why you will see one-way streets in many downtown areas – it is the most efficient way to move large amounts of traffic with relatively few lanes. However, as we get away from downtown areas, one-way streets are much less prevalent. Coordinating signals on two-way streets where the signals are irregularly spaced is a much more difficult challenge. In some cases, it becomes physically impossible to provide good coordination simply due to the spacing of the intersections and the speed that people are driving.
To provide signal coordination, most signals in City of Lakeland are wired or connected to the central computer at Traffic Engineering through a web of underground cables. The central computer is what coordinates the signals through constant communication by telling them when they should turn green in order to allow for a stream of green lights along an arterial while traveling at the speed limit. The central computer also allows for longer green lights when traffic is heaviest!
When the communication between a signal or series of signals is disrupted or cut off from the central computer, those signals continue to cycle, but they are no longer coordinated because they have nothing to tell them when they have reached their strategic moment to turn green; therefore, the signal is operating completely independent of the other signals around them. Communication disruptions usually occur from a cut in the underground cable by some construction crew, through “noise” in the line or by a pre-emption.
Cuts in the underground cables are the worst type of communication failure because they take the most time and money to restore or repair and they also disrupt all other signals downstream from the cut. Cuts may affect as few as one signal to as many as several hundred!
“Noise” in the line usually disrupts communication to one or two signals in an area. Think of a computer connected to the Internet via a modem (on-line). If there is too much static or noise in the telephone line, that modem could disconnect or fall off-line. This type of communication failure is usually corrected within 24 hours. Traffic Engineering generates a report each morning that identifies all of the signals that have fallen off-line and communications experts are dispatched to those signals to restore the communication.
Pre-emption occurs at signals near railroads, fire stations and drawbridges. A pre-emption causes the signal to immediately ignore the central computer, utilize a special cycle sequence, then return to normal operation. Pre-emptions are self- correcting and do not require Traffic Engineering personnel to restore communication.
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3. There is not enough pedestrian time to cross the street! The pedestrian signal does not work. |
Every pedestrian signal has a WALK followed by a flashing DON’T WALK. Frequently, citizens will report a signal that does not allow enough time to cross the street on the WALK.
The WALK signal means to begin crossing the street and watch out for right turn on red motorists (although the pedestrian legally has the right-of-way, many motorists are not watching for pedestrians). A flashing DON’T WALK means that if one has already begun crossing, continue! The flashing DON’T WALK should provide enough time for most pedestrians to safely cross the street. Flashing DON’T WALK also means if one has not begun to cross the street, then wait until the next WALK appears after pushing the pedestrian button.
Example 1:
The walk light does not appear or is too short to cross Federal Highway.
Answer: First of all, that pedestrian signal will not appear until a certain point during a signal cycle. Pedestrian signals do not make a signal appear any quicker than a concurrent vehicle movement. Once it appears, a WALK of at least 5 seconds indicates to the pedestrian to begin walking; then a flashing DON’T WALK occurs. This means if the pedestrian has begun, complete the crossing. Pedestrian signals EXTEND the concurrent green to the appropriate amount of time needed to cross a street. NEVER rely on a green light indication for it may be insufficient.
Example 2:
The WALK light stays on for a long time and button never has to be pushed.
Answer: When signals are in a coordinated mode, they rest in WALK in the direction of the coordination; there is no reason for them not to. The reason a button has to be pushed to cross the major street is that less time is usually needed for vehicles to cross than pedestrians. By activating the pedestrian signal, a fixed amount of crossing time is guaranteed.
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4. There should be a delay before the light changes to the next movement, too many people run red lights. |
Many of the signals in City of Lakeland already have a delay and in the near future they all will. This delay is known as ALL RED and means the signal is red in all directions for a set amount of time until it cycles to the next movement. The amount of ALL RED is based on the speed limit and the distance a vehicle must travel through the intersection.
Casual observers will not notice the ALL RED but it is noticeable if one looks closely. The ALL RED is designed to get a vehicle through the intersection traveling at full speed that simply does not stop. If too much ALL RED is applied to a signal, then it becomes obvious to motorists and more people will run the red light. Also, remember that if we add four or five seconds of ALL RED, that means four or five seconds less green. Many of the signals in City of Lakreland’s busy street system simply do not have much green time to spare.
Never assume an intersection is safe just because it may have ALL RED. There is always the possibility someone may run a red light at any time! Furthermore, always look once the signal turns green before proceeding through the intersection.
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5. The signal cycles even when no vehicles are present. |
Every signalized intersection has a designated major street or movement, usually the through movement of the major street. All other movements are actuated or on demand. In order to have actuation, some sort of vehicle detection is necessary.
In City of Lakeland, the most common form of vehicle detection is a magnetic induction loop connected to a detector. This is simply a wire loop embedded into the pavement of each lane at the stop bar that extends back 20, 40 or 60 feet. When a vehicle is over the loop, the signal for that movement is "triggered" or called. Once a green light appears, the signal will stay green as long as it needs to up to a set maximum amount of time. If only one or two vehicles are present then the green may last for 6 seconds or so. If 15 vehicles are present then it may stay green for 30 seconds or so. If no vehicles are present, the signal will not cycle to that movement.
Unfortunately, asphalt pavement is relatively soft and loops are very fragile so they often fail. When this occurs, the signal is set to cycle to that movement all of the time for an amount of time consistent with the amount needed during heaviest traffic.
Fortunately, City of Lakeland and Florida Department of Transportation has been moving toward other non-pavement invasive types of vehicle detection. Among these are microwave and video detection.
Microwave detection works similar to motion sensors utilized with burglar alarm systems. Microwaves place a call for a signal as long as it senses moving vehicles.
Video detection works by continuously taking a "picture" of an approach lane then comparing it to the previous picture. As long as the picture is continuously different than its previous one, the signal will stay green once it begins.
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6. The light is too short, it needs more time! |
Usually one of the following three things are occurring;
(a) there is too much traffic,
(b) someone is not paying attention, or
(c) a pre-emption.
Example 1:
There is too much traffic. All roadways have a certain capacity and once this is exceeded, the traffic signal cannot clear each movement as is normally expected. US 98 and Griffin Road is one such example. Traffic continuously approaches all directions at that signal during rush hour. All of the movements are given a generous amount of time but it is never enough to clear all of the vehicles in any direction. If any of the movements at that intersection are delayed any longer than they already are, the backup grows exponentially. Overcapacity is usually a temporary condition.
Example 2:
Someone is not paying attention. Evon is the fifth vehicle in line waiting to turn left from Edgewood Dr onto S. Florida Av. Normally, the green arrow has enough time to clear up to 12 vehicles but this time the driver of the vehicle in front of her sped up after initially not realizing the light was green and made it through the yellow leaving Evon stuck until the next light.
Answer: The driver of the last vehicle that made it through the light was not paying attention causing too much time to elapse between vehicles passing over the detection zone. The signal sensed no more vehicles so it turned yellow.
Example 3:
Rick is eastbound on I-4 and exits at US 98. Ten seconds after the light turns green it suddenly turns yellow when normally, the light is green much longer due to a high number of vehicles waiting to get off of I-4.
Answer: This interchange has a priority premptor that gives special priority to emergency vehicles. Whenever a fire/ambulance passes, the signal is pre-empted and immediately follows a special sequence designed to give piority to US 98. Once the fire truck/ambulance passes, the signal returns to its normal sequence and the eastbound off ramp will then get its normal amount of green time.
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