Saturday, July 12, 2008

Synchronization of Traffic Signals in The Woodlands Texas


Have you ever wondered why the traffic light changes from red to green, sometimes just before you get to the light? It is probably because we use traffic-actuated signals here. In The Woodlands, actuation is triggered by vehicle detection, pedestrian request, fire truck request, and timing. However the computerized decision to change the light is not as simple as it sounds.

Traffic Intersection System
Each signal location has several cameras, a control box and of course the traffic lights themselves linked to a control box. Attached electronically to the intersection control box, each camera monitors a street location consisting of one or more traffic lanes.

As you might notice, the right turn lane in one camera is not monitored and the other is monitored. That is because it is a right turn lane, assumed to not be a factor for changing the traffic signal in one, but in the other lane, an auto will affect the flow of the traffic. The boxes are imaginary boxes, nothing physical on the ground. The controller software monitors the rectangular boxes to detect an automobile. On the central control management display at headquarters and on the precinct’s web site, these boxes change color when the intersection controller detects an automobile. That color change indicates the control box has detected an automobile in the intersection. One way to check on the operation of the control system is for the traffic manager to observe these changes and see that the control system is responding properly to the movement of traffic.

The control box has a computer “brain” which is programmed to react to traffic presence situations. For example, let’s take an intersection on the Woodlands Parkway. If a car is traveling in the right hand turn lane, it is assumed to be turning to the right. If in the middle lane, the assumption is that it will pass through the intersection. For the left lane, the auto is assumed to be passing through depending on the lane configuration. And in the far left turning lane, the vehicle is assumed to be turning left. So here comes an auto in the right lane on the Woodlands Parkway and it moves to the right turn lane of the intersection. A perpendicular traveling auto is waiting for a green light at the intersection. That car now has been given priority by the computer at the intersection to receive a green light, because of these assumptions. In this case, without another automobile to consider, the computer changes the light to green for the waiting vehicle, that is, if synchronization and wait time criteria are also met. The determination by the computer to change the light is relatively simple, but is governed by several variables.

All fire trucks have priority for a green light at intersections. The truck emits an infrared light beam to tell the management system it is approaching the intersection. The system immediately responds by ensuring the light is green for the truck and red for the perpendicular traffic. A fire truck is very heavy and difficult to stop in an emergency. If it is slowed, the truck is very slow to clear the intersection because of its slow acceleration capability. It needs to proceed out of the intersection quickly so as to not impede the movement of other traffic. Police cars and ambulances are not given priority. They can move through the intersection and accelerate on by, quickly getting out of the intersection even if they have to stop.

Each intersection control box is attached to an optical fiber cable to communicate with the central control monitor and control system. Many problems with the intersection equipment can be fixed from the central location through this connection. Camera images are sent to the control center at five frames per second. In the intersection control box, the computer is able to detect automobiles in the intersection after the light changes and is able to determine the speed of a vehicle. It is able to react to these observations by signaling a photograph camera to record the event, if so programmed and the required still cameras are installed. The control system also counts vehicles moving in each direction.

Traffic Signal Synchronization
Woodlands Parkway is characterized as a “Smart Street”. All traffic is considered in optimizing vehicle flow from one point to another, and the perpendicular traffic is allowed to satisfactorily move also. This includes the pedestrian and emergency vehicles. How does the synchronization of a series of lights occur? We should know this if we are to drive through the synchronized signals without stopping.

First, the time between any given intersection and the next signal light must be programmed into the controller. The optimum velocity is utilized to time the arrival of incoming vehicles from that signal light. So typically, a velocity on Woodlands Parkway would be assumed to be the speed limit, about 45 MPH. This information usually resides in a data table. The optimum time for changing the light is programmed into the control box from that sheet. If an automobile has a reasonable acceleration from a dead stop at one traffic signal, he should arrive at the next signal light in time for a green light, at is, if he travels at 45 MPH. If he travels too fast, he arrives too early and has to stop. That may cause automobiles behind him to stop or slow down as well, so he has caused a potential bottleneck in the traffic flow.

An example of this – East Panther Creek/Woodlands Parkway intersection is considered one of the two key intersections for successfully optimizing the movement of traffic on the Woodlands Parkway. We want it to be green when we get there. We are at Six Pines heading towards East Panther Creek, passing through the light without stopping and hope to pass through the light at Panther Creek in the same fashion. That is a reasonable expectation except there may be many cars in line waiting to be released by a green light at the next intersection. Vehicles are being fed onto the parkway at Grogan’s Mill. The light will turn green in a certain number of seconds, because it is synchronized with the signal at Six Pines. If the traffic rushes to Panther Creek and stops, that traffic will not be traveling at 45 MPH for several seconds after the light turns green. The Traffic Manager has programmed this additional time into the control unit. It is assumed that a certain number of automobiles are being added to the traffic lanes from Grogan’s Mill. Now the control system at the Panther Creek intersection is told that the traffic has been released at Six Pines. The control box starts its countdown for target signal change. While it is doing that, it detects 15 cars passing through the two lanes at the intersection within the last 25 seconds. The entire situation becomes difficult to interpret, but the system has been programmed to prioritize the synchronization over the vehicle detection rate, so it goes ahead and gives the Woodlands Parkway traffic a green light after giving a red and momentarily pausing for the perpendicular traffic to clear. So the light is green for us by the time we arrive. Had a pedestrian pressed the cross walk button, we would have had to wait because the pedestrian had to be serviced within a certain time. Had a fire engine come along, we would have also had to wait. The reality of synchronization is that it may work for us sometimes and other times not. Many times, the traffic flow just cannot be predicted.

All of this becomes more complicated as we have more or less traffic entering the roadway from other locations. You can see some of the logic in the computer and how difficult it must be to get it right. The term synchronized must be regarded as a process, not an actual real thing that we can rely upon. The process only helps the traffic to flow better, not necessarily keeping every car moving from one end to the other.

If you are traveling on one of the "Smart Streets" of The Woodlands at night and wonder why you have stopped at a red light, it is likely that cross traffic triggered the signal. Synchronization is not intended for off hours. The process is intended for peak hours when traffic needs to be moved most efficiently. Soon we will add Research Forest and Lake Woodlands to the smart street inventory. Currently, only the Woodlands Parkway is synchronized.

Precinct 3 is now studying our major traffic arteries to seek better timing of the lights in the Northstar system. Lake Woodlands Drive and Research Forest will be synchronized when the traffic signals are functioning. Lake Woodlands is likely to become a 40MPH drive, higher than current limit of 35 but lower than the first predicted 45MPH, except up near the mall where it will remain 35MPH. Many thanks to both the Commissioner Ed Chance and the Traffic Control Manager Fred Koehler, for explaining how the system functions. Related short articles to be released soon:

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