 Traffic detector failures cause serious delays in traffic movement and increase maintenance costs. Proper detector design and installation will significantly increase detector efficiency. However, it is impossible to eliminate all detector failures. In this section of our videotape, we will discuss the most common failures, ways to troubleshoot problems, and basic detector maintenance procedures. Traffic detector failures are described in terms of their effect or the problems that they cause. For example, a failure that causes a traffic signal to skip one phase is referred to as an omitted phase failure. Other common failures include a stuck signal, a phase extending to maximum, intermittent problems, crosstalk, and splash over. An omitted phase is usually caused by a loop failure. Older versions of detector units would fail in the open position, resulting in no calls to the controller. Newer detector units have circuitry that provides a continuous call if the loop circuit is incomplete. A stuck signal may be caused by a vehicle not being detected, or by a detector with a delay feature failing to retain the call long enough. In some instances, however, a stuck signal may be caused by a controller failure. Phase extending to maximum is a situation that is usually due to a continuous call from the detector unit. This call causes a specific phase of the signal operation to extend to the maximum time set on the controller, regardless of the traffic demand. A faulty detector unit or an open loop circuit may cause this continuous call, which increases traffic delays. Intermittent problems can develop for a number of reasons. Low sensitivity of the loop, the detector unit, or a combination of both can cause intermittent problems. Unstable oscillators in the detector unit can cause problems during rapid temperature shifts, but many can be retuned. A broken loop wire can reconnect during pavement shifts, causing intermittent problems. External devices installed for lightening protection can be damaged by a power surge and cause these problems. Finally, ghost problems, an intermittent operation breakdown for no apparent reason, can require the replacement of a detector unit. Cross talk is another type of detector failure occurring when the frequency of one loop installation is influenced by another. This frequency lock between two oscillators can be caused by poor quality connections in the loop system. Field coupling due to the closeness of two loops, coupling between two closely spaced lead-in wires, coupling between lead-in cables sharing the same conduit, or the coupling between closely spaced harness wires from the field terminals to the detector. Cross talk can produce a false detection when there is no vehicle in the detection zone. It can also result in a lock-up after a vehicle detection. Splishover is the false detection of vehicles outside of the detection zone. It occurs between lanes controlled by different phases. Occasionally, the loops are simply placed too close to the lane line. Usually, however, splishover occurs when long loops are operated at high sensitivity in order to detect small vehicles such as motorcycles. Surveys of traffic agencies show that there are eight primary causes of loop failure. Pavement problems such as cracking and moving, breakdown of the wire insulation, poor sealants or incorrect sealant application, inadequate splices or electrical connections, construction damage, improper detector unit tuning, detector unit failure, and lightning and electrical surges. Many agencies report that installation errors cause the majority of their maintenance problems. These errors can run from sloppy installation and poor inspection to the use of low-grade components. Agencies who contract out their detector installation should apply strict pre-qualification guidelines to avoid awarding the contract to an inexperienced contractor. Next, these agencies should inspect the contractors' work carefully to catch any errors or potentially troublesome shortcuts. For example, using improper tools to install the loop wire might cause detector problems that won't show up until months after the installation is complete. Developing an active inspection and maintenance program can make a difference. The Illinois DOT program is so successful that no more than 5% of their loop detectors are inoperable at any given time. Troubleshooting a detector failure calls for a little detective work. First, verify whether the problem is actually with the detector system. Often, a visual examination of the system reveals the problem. Otherwise, it is necessary to systematically troubleshoot the system. An experienced technician begins his maintenance work by inspecting the roadway around the saw cut. Is the pavement cracked? Have the wires floated to the top of the sealant? If the wires are exposed, damaged or buried too shallow, the loop installation should be replaced. Occasionally, the sealant becomes cracked or deteriorates. This is a problem for agencies in states that frequently salt the streets and use snow removal equipment. Unstable sealant should be removed. The slot blown clean with compressed air and a new sealant applied. Potholes within the loop area should also be repaired. Some agencies use a cold mix compound to patch the pavement. However, this should be considered a temporary measure. To decrease detector failures due to these loop wire problems, visually inspect saw cuts every six months. Another maintenance situation occurs when a loop cannot detect a vehicle due to insufficient sensitivity. Usually, the loop's inductance is too low. And the technician can measure this with one of a number of test meters. Low inductance can be caused by too few turns of wire, the shorting together of the wire turns, or the steel mesh in the road reproducing a shorted turn effect. In a multiple loop configuration, the defective loop can be eliminated, which temporarily corrects the problem. Or consider connecting multiple loops in series or series parallel to maintain an acceptable inductance. Cross talk can be a particularly bothersome maintenance problem. Your handbook offers detailed information concerning this maintenance headache. Let's take a look now at a few of the common cross talk causes and solutions. First, poor quality connections in the loop wiring can cause cross talk. The solutions focus on upgrading these connections, such as using high quality soldered joints. Physical closeness of two loops can cause cross talk through field coupling. Consider frequency separation or the use of multi-channel multiplex detectors to solve this problem. Two closely spaced lead-in wires coupling can create cross talk. Solutions include routing the wires in separate slots and twisting the loop wire pairs a minimum of five turns per foot. Coupling between lead-in cables sharing the same conduit can cause cross talk. This problem can be corrected several ways including the use of individual shielded twisted pair cables. A potential solution is the three terminal method described earlier that requires the grounding of each lead-in cable separately. The coupling of closely spaced harness wires from the field terminal to the detector can create cross talk. Using twisted wire pair or using shielded twisted cable with proper grounding is the solution. A general procedure for identifying which detector unit is cross talking begins with the disconnection of all detectors except the suspected unit. If no new false calls or lockups occur then the cross talk is coming from another unit. Reconnect one unit at a time until the cross talk begins again. Once the problem unit has been identified it can be repaired. Once you have eliminated loop problems and cross talk as the basis for a detector failure consider replacing the detector amplifier unit. The substitution of a new amplifier for a malfunctioning detector is possible if both units have the same characteristics and quality. There are several operational checks you can conduct before replacing a detector unit you suspect of malfunctioning. Using a maintenance vehicle and a vehicle simulator you can analyze adjacent lane detection, motion in the loop wire, intermittent detection and the sensitivity of the loop system. Adjacent lane detection is a problem associated most often with the use of long loops such as those used in left turn lanes. To test a loop maneuver your maintenance vehicle close to the lane. Monitoring the detector unit adjusts the sensitivity to stop the loop from detecting the maintenance vehicle. Then check the loop to ensure that it can still register small vehicles in the detection zone. For details on other solutions such as changing loop configurations consult your handbook. Intermittent detection problems can develop from a number of malfunctions. To identify the cause drive your maintenance vehicle over the loops you suspect are malfunctioning. These intermittent problems should appear if you repeat the test several times while monitoring the detector unit. Another technique requires visually inspecting all connections and testing the system with a 500 volt mega and a low ohm mid-scale ohm meter. Electrical as well as operational tests can be performed to help identify the cause of a traffic detector malfunction. Basically there are four devices used to analyze loop systems. A mega or high voltage resistance tester for measuring insulation resistance. A frequency tester also called an inductance meter for measuring inductance. A digital volt ohm meter for measuring series resistance. And a low ohm mid-scale ohm meter for measuring the dynamic change in series resistance. Both low resistance to ground and high series resistance are major factors in loop system malfunctions. Low resistance to ground can occur for several reasons. For example it can result from poor insulation on the loop or lead in wires. From the inadequate ceiling of splices or from exposed wires. High series resistance can be caused by poor splices, corroded or loose screw terminals, poor crimping or incorrect wire size. Detectors with a high series resistance problem are sensitive to humidity, temperature and vibration. Both resistance problems can cause drifting and false calls. Refer to your handbook for a table detailing these and other common loop malfunctions and their probable causes. As we have discussed identifying the source of a loop detector malfunction or failure can be an involved sometimes difficult process. In this section we have covered several ways to troubleshoot loop detector system problems and isolate their causes. To summarize and to simplify the maintenance process we recommend performing step by step these following tests during a maintenance call. Step one, visual inspection. First check for damaged loop or lead in wires. Then look for open leads within the controller cabinet as well as the availability of power to the detector. Step two, check the detector unit. If you suspect a problem replace the detector unit with a similar functioning unit. If this does not improve the system's operation reinstall the original detector unit and proceed with the next test. Step three, measure the Q factor. Measure and record the information requested on the quality or Q data form. For example measure the resistance from either loop terminal to the bus or conduit ground. Record this value on the form. Then check for series continuity between the terminals and so on. Step four, determine Q. Following the procedure described in your handbook determine the Q factor of the loop system and record the results on the Q data form. Step five, measure the sensitivity of the loop system. Using one of the two methods detailed in your handbook measure the system's sensitivity. Then record these frequencies on the sensitivity or S data form. Step six, analysis. Compare the values for each of the system characteristics with the acceptable limits listed in the loop troubleshooting chart in your handbook. Then take the table's suggested corrective action to repair the detector problem. Now let's take a brief look at the maintenance of the micro loop. To operate efficiently the micro loop detector system requires the same installation precautions as the loop detector system. For example the micro loops probe cable should be inserted in the slot and sealed with the same care as a loop wire. And the micro loop splices should be as sound and as protected as the splices in a loop system. Failure however can occur in any of the micro loops subsystems. There are four key areas that affect the operation of a micro loop detector system. Probe burial depth, probe movement, probe cable and saw cut maintenance. The proper depth and placement of a probe is critical to the system's performance. A deep placement provides good single count detection but results in low signal levels. A shallow placement provides a stronger signal but a higher risk of double counting. Therefore it is important to match the probe placement with the site and intended application. The movement or lack of stability of a probe in the pavement can affect detector performance. The probes must be firmly supported in their holes. Use a PVC conduit as a shell and tamp sand around each probe to prevent any lateral movement. The probe cable must be a water block type of cable to prevent moisture damage. For example, moisture can cause excessive capacitance or leakage between the wires and to the ground. And moisture across the leads of the micro loop can induce drift. Wherever possible run the probe cable directly without splices to the detector unit. If splices are necessary ensure that they are electrically sound and environmentally protected. Saw cut maintenance should include a visual inspection every six months to evaluate the sealant and the pavement. Any cracking or damaged sealant should be chipped away, blown clean and replaced with new sealant. If a micro loop detector system malfunctions or fails try the following troubleshooting procedures. First visually inspect all the elements of the system for problems. If none are apparent examine the probe for tilting. If due to pavement movement the probe has tilted pull and reinstall it properly. If there isn't a tilting problem check the cable, wire, probe, splice and detector unit. Systematically run a test on each element to determine the source of the malfunction. Compare the measurements from your tests to the measurements on the original reference sheet. Note the new test values on the reference sheet when you have completed the repairs. Most magnetic detectors have an extremely good maintenance record. The opportunity for failure is small for two reasons. The detectors are installed under the road surface and all the lead-in wires are encased in an underground conduit. Some magnetic detectors have been installed for more than 20 years with no failures. Although magnetic detectors are very reliable please refer to your handbook for ways to troubleshoot any problems that may occur. In the different sections of this videotape we have examined the detector principles, theories and evolution. Common detector applications, the criteria and considerations of detector design, basic detector installation techniques and the fundamentals of detector maintenance. We recommend supplementing the information in this videotape with your traffic detector handbook and other Federal Highway Administration reference materials.