PLC Applications In Traffic Signal Operations

Written by Tony Rucker and Jessy Pu, City of Campbell

What’s a PLC?

Generally speaking, a Programmable Logic Controller, or PLC, is a solid state control system which has a user-programmable memory for storage of instructions to implement specific tasks, such as: I/O control logic, timing, counting, arithmetic, and data manipulation. PLC’s are generally programmed in what is known as Ladder Logic. This method of programming was established because it could be closely related to hardwired relay logic that PLC’s were developed to replace. PLC’s are used in many industries for process control such as parts manufacturing, lumber mills, food processing and auto manufacturing.

The City of Campbell uses PLC’s to supplement the operational needs of several signalized intersections and were chosen because of their high performance-to-cost ratio over relays and 24 volt external logic cards. The PLC receives inputs from the signal controller’s NEMA outputs, makes logic decisions based on its operator-written stored program, and then outputs commands to the signal controller’s NEMA inputs. The PLC used in Campbell for traffic signal operation, the “IDEC Micro-1® “, is a fixed, 8 input/6 output, “brick type” PLC that can be expanded to a total of 16 inputs and 12 outputs. Although several I/O types are available, the model used in Campbell has “source” inputs and “sink” outputs, so that like a NEMA signal controller, it recognizes a ground as a “true” input and outputs a ground as a “true” output, in reference to the cabinet’s +24 volt dc power supply. It has EEPROM memory capable of storing 600 steps of user program and numerous internal logic components such as “AND” gates, “OR” gates, latches, 80 timers, and 48 counters. It can be programmed with either a hand-held “Boolean type” loader with LCD display or ladder logic software that runs on an IBM, or equivalent computer. Other PLC models and brands are available that can be modularly expanded as needed to provide up to 512 I/O points, floating point math, high speed counting, line voltage I/O and analog I/O. These more costly, higher-end units can be configured with RS-232/422/485 serial interfaces for peer-to-peer networking and telephone modem interfacing, providing remote control and monitoring. Of course, the concept of using external logic cards has been available for some time from controller manufacturers, e.g. “Econologic”, “MultiLogic”, etc.. However, this PLC only occupies 1/4 cubic foot of space and for only $300 it can replace relays and timers costing 10 times as much and taking up 50 times more space.

A Sample PLC Application in Campbell

The intersection of Winchester Blvd. and Budd Ave. has a 6-phase signal with high left-turn/right-turn overlap traffic and pedestrian/right-turn conflicts. Figure 1 shows the signal phase sequence and intersection configuration. The goals were to provide the right-turn ( 1+ 4) overlap operation while protecting pedestrians from the right-turn traffic when they are in a crosswalk. The operation was designed with the following logic: turn on the right-turn green arrow to overlap the left-turn traffic ( 1 or 4) when there is no pedestrian conflicts, and stop right-turn traffic (red ball) when pedestrians cross Winchester Blvd. ( 4). In order to accomplish the operational objectives, a PLC was utilized to perform the logic decisions when the overlap is allowed and also detector switching and delay for the right turn lane ( 1+ 4). The following is a brief description of the programmed logic in the PLC:

When 4 turns green and there is no concurrent pedestrian service, the overlap right turn green arrow will be displayed along with a green ball on the same signal head. If there is demand on 1 at the time 4 terminates and 1 is next, the overlap right turn green arrow will be maintained while a yellow ball on the same signal head is displayed for 4. A red ball is then displayed for 4 when the 4 yellow times out. This overlap right turn green arrow will remain on as long as 1 is green. When 1 terminates and goes to yellow, the overlap right turn green arrow then is replaced by an overlap right yellow arrow.

If there had been phase pedestrian service at the start of 4 green, the overlap right turn green arrow and overlap right yellow arrow will not be allowed by the PLC. It will not be allowed during 1 if not allowed during 4. In other words, the overlap right turn arrows are allowed only at the beginning of green for 1 if 4 is skipped or at the beginning of green for 4 and no concurrent 4 pedestrian service.

The cabinet wiring was modified and the Conflict Monitor was programmed to additionally protect the 4 walk from the overlap right turn green arrow. The PLC, besides controlling the signal controller’s overlap output, also provides some detector switching and delay logic functions. Its operation is as follows: When a vehicle arrives on the 4 right turn loop and 4 is not in green, a timer in the PLC starts. Upon time out (8 seconds), the PLC calls 1. If 2 terminates and there is no demand on 4’s left lane, then 1 is served, concurrently with the northbound thru 6, and the overlap right turn green arrow comes on. During this 1 green, vehicles on 1 or the east to south right turn can extend 1. If, however, 4 is green because of demand for service in the left turn lane of 4, then vehicles on the right turn will call and extend 4 if there is no demand on 1. If right turn traffic is heavy and there is demand on 1, then 4 can gap out if there are no left turners and the overlap right turn can remain on as 4 is terminated and the actual demand on 1 is served.

Another Distinct Advantage

Another distinct advantage of PLC’s is that they can be easily modified or reprogrammed to meet changing intersection operational needs without having to purchase and install more connectors, sockets, cards, or relays. Usually the required modifications are limited to running a couple of wires between I/O points of the NEMA controller and the PLC. After the new logic operation has been checked thoroughly in the shop for the correct operation, the technician can download the revised program in the field with a loader or a laptop PC and then field check the operation to insure its conformance. However, if the PLC-to-Cabinet wiring is installed as described below, in a matter of seconds you can replace the existing PLC with a spare PLC already pre-programmed with the revision.

PLC Wiring in Controller Cabinet

PLC’s are generally hardwired in their more familiar process control environment such as plants and factories. This is generally not acceptable in the realm of traffic signals, as technicians prefer the modular, connectorized concept of the controller, conflict monitor and other cabinet equipment applied to all active components. This facilitates maintenance and decreases down time. To achieve this, the City of Campbell selected a 24 pin “AMP” brand circular plastic connector (CPC) cable system. The chassis-mounted plug with male contacts is mounted on a small piece of sheet aluminum that is mounted with stand-offs to the PLC’s mounting holes. Standard nylon-jacketed, 19 strand #22 AWG wire is used to connect the pins from the rear of the CPC to the terminal points on the PLC. Only the AC power, neutral and ground should be carried on shielded #18 AWG cable. A mating CPC receptacle with female contacts is wired with conductors of sufficient length to be connected to the appropriate controller I/O terminals in the cabinet. A 35 mm DIN rail is secured to the cabinet’s inside wall where appropriate and then the PLC is snapped in place. Terminal blocks, fuseholders and relay bases are also available that are specifically designed to snap onto the DIN rail, further facilitating the process. Installation is completed by screwing on the female harness to the PLC. A standard wire list of PLC I/O function assignments was adopted early and adhered to, so that PLC’s are interchangeable throughout the City. A PLC can be removed and replaced with another unit in less than 30 seconds. The only unique feature of the PLC installation at a site is the PLC’s internal logic program.


This article describes the use of reasonably priced, readily available alternative control tools to enhance the signal operation beyond what a typical NEMA signal controller offers. A PLC application has been successful in Campbell but it should be noted that, to successfully program and install a PLC requires above-average knowledge and experience in NEMA controller operational specifications in addition to PLC programming. An experienced signal technician can obtain the knowledge required to implement PLC operation by taking courses in PLC operation and Ladder Logic Programming, beginning with basics and fundamentals and then graduating to the more advanced classes. Most major manufacturers offer factory courses and many are free. Contact the manufacturer’s representative for the PLC of your choice for details.

It would be ideal if a signal controller had a user programmable area for a traffic engineer or signal technician to program the desired logical operation without having to rely on external devices. But until that time arrives, you might consider using a PLC to enhance or optimize your signal operation. There are four PLC’s currently in use in Campbell. They have provided continuous, trouble-free operation since installation in 1994.

Loop Configuration Evaluation

Written by Don Wood, County of Santa Clara

An evaluation of the effectiveness of various forms of vehicle loop installations and configurations. In 1990, the County of Santa Clara made an evaluation of the effectiveness of various loop detector configurations for the detection of bicycles. It was desirable to provide bicycle detection for each lane at each intersection. There were all sorts of claims regarding the effectiveness of various loop configurations being made. This evaluation conducted by the County of Santa Clara was not extremely detailed nor exhaustive. It did provide an indication of how these types of loop installations would work as they are typically installed by the County.

Our standard loops being used at that time were Type “A” (6×6′ hex loop). There were a few locations with Type “Q” loops installed as the front loop of an array of one Type “D” and two Type “A” loops. Our experience with that configuration was not proving successful.

We prepared test loop fixtures of type “A” loops, type “D” loops and a type “E” (Round) loops and a type “B” (diamond) loop. Identical tests were performed on each loop in “solo” and as part of an array including the test loop and two Type “A” loops connected in series. Measurements of the sensitivity of the various configurations when presented with the influence of lightweight bicycle wheel were recorded. In addition, a measurement of the height of the detection field was made by suspending a metal object over the loops and raising the object until the actuation was dropped by the detector.

There were three basic questions that we were seeking to answer:

1. What was the sensitivity of the various loop types to small metal objects (bicycles) within the loops detection area?

2. What was the effect of combinations of various loop configurations on the sensitivity of various loops?

3. What was the range of detection height of the various loop configurations.

The County typically uses area detection as part of it’s signal installations. These are usually three six by six foot Type “A” loops centered in the traffic lane starting three feet behind the stopbar and spaced ten feet between the loops (16 foot center to center). They were normally all three turn loops except the head loop of the left turn lanes which were normally four turn loops. We were trying to improve our detection without requiring the replacement of all loops. Our intent was to try to install new front loops that were bicycle sensitive while still maintaining proper detection of larger vehicles.

Four loop configurations were tested.




Type “A”





Type “B’


3 and 5

Type “D”

3 and 5

Type “E”


3 and 4

Test #1

A bicycle was rolled through the loop area at three different positions. At the center, at a point approximately 18 inches from the left edge (45 degree), and at the left edge. At each position, the highest variation in loop frequency was noted and the percentage of deviation was calculated. This test was performed twice, once on the loop connected to the amplifier in “solo” and a second time with the loop connected in series with two Type “A” (standard 6×6) loops.

The results of this test indicate that the type “A” loop, when connected in solo has the greatest amount of deviation. In combination, the Square 3 turn loop provided the greatest deviation, however, the Type “A”, the Diamond, and the Round 4 turn combinations were close.

Test #2

A metal object was placed over the loop area and the deviation in loop frequency was noted at various heights and the point of loss of detection was noted. Loss was indicated in feet above the pavement.

From this test it is apparent that the Type “B” (diamond) 5 turn loop performed best overall followed closely by the round 4 turn loop. The type “D” 5 turn loop performed the best at the first level (0 feet), but demonstrated low sensitivity at all other heights.

Another factor in loop performance is loop life. Loops can fail prematurely due to pavement failure around the loop. The extent of damage to the roadway can be reduced by reducing the number and extent of cuts to the roadway in particular eliminating closely spaced cuts. Using preformed loops or paving over the newly installed loops are also effective means of reducing the pavement failure around loop installations, however, this is not always practical especially in a replacement situation.

Considering the results of both tests, and the methods of installation, the performance of the type “E” (round) loop is very good and the roadway damage very minor. The performance of the type “B” is very similar to the round loop with possibly a more even sensitivity across the entire width of the loop. The type “A” loop is also very sensitive and has good detection height, however, they do not provide even detection across the entire width of the loop. They do require more pavement cuts and patching and therefore cause more damage to the roadway. The type “D” loop configuration’s main advantage would be the generally even sensitivity throughout the area of detection. However, it is not as sensitive nor does it provide good detection above one foot in height. The damage caused to the roadway during installation of the Type “D” loop is the greatest of any configuration.

The depth and number of cuts involved in installing each type of loop is extremely different.

The depth required for a round loop is only that required to cover the four loop conductor turns plus the required filler height. Two and one half inches is sufficient. The Type “D” loop requires approximately three inches throughout the loops pattern and approximately five inches at the point of intersection of the homerun. In addition, the type “D” presents the danger of large pieces of AC breaking out due to the intense cut pattern. Even with the intense cut pattern, there are still some severe angles presented to the loop conductor path. These angles may lead to early loop failure. The type “E” (round) loop has only one intersection of the cuts and is cored to improve the turn radius at the exit in order to reduce the potential of loop failure. The type “B” (diamond) loop requires the next least number of cuts when it is cut with square corners that are cored.

The height of detection is important due to the passage of high axle vehicles and the fact our signals rely on good detection in order to extend the timing of the phase. If a truck or 4×4 vehicle were to pass slowly through the lane, the phase might terminate prematurely. The loops with large center areas have higher detection height (over four feet high for the round loop). Loops with closely spaced runs have lower detection heights. Roughly one half the distance between sides of the loop is an indication of the reliable detection height.

From the above tests we determined that the type “E” (round) or type “B” (diamond) loops provide the best detection capabilities for the vehicle and bicycle detection needs of the County. They have excellent sensitivity, a very good detection pattern including the detection height and bicycle sensitivity. And the potential for pavement damage is reduced significantly by use of coring.

Both the round and diamond loops may also present less adjacent lane pickup even with higher sensitivities due to the edge alignment of the loop being 45 degrees to the adjacent lane. More of the loops most sensitive areas are located farther away from the edge of the lane. The diamond loop may be best in this regard due to its straight edge at 45 degrees.

The original data from these loop tests is no longer available. This article is taken from a report that summarized the test results. The report was prepared by Don Wood using data collected by Tony Rucker and Stuart Leven. Our standard loop installation was revised as a result of these tests to be either round or diamond loops in an array of three loops spaced 16 feet center to center with the front loop having 4 turns and the middle and rear loops having 3 turns, all connected in series. The type”B” loops are installed without corner crosscuts using coring instead. We have found this configuration to meet our requirements for vehicle and bicycle detection.