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Portable Traffic Signals in the Work Zone
by Arlen Yost & Lyle StoutIntroduction
Work zones are a fact of life as our society attempts to maintain and improve our transportation infrastructure. As such, we must also recognize that there are always four elements in tension in the design and execution of a work zone – minimizing the delay and disruption of the traveling public, maintaining the safety of both the public traveling through and the workers within the work zone, containing the costs to the public, and maintaining a margin of profit for the contractor doing the work. Traffic control within the work zone has a direct effect on all of these elements.There are many types of work zones, and many types of traffic control within work zones – more than this article could adequately address. Using portable traffic signal equipment to minimize delay, increase work zone safety, control costs and improve profitability is the focus of this article. We will also look at the past, present and potential future of portable traffic signals. In this article, portable traffic signals (PTS) are defined as traffic signals and associated control equipment that can be quickly and easily transported and deployed into a work zone. The collective group of these devices makes up a system that is able to control traffic in an orderly, safe manner using standard red/yellow/green traffic signals.
Common Applications
The single lane road closure is a large part of the portable signal marketplace. These simple systems are designed for fast deployments with simple to use control equipment. This equipment is and should be oriented for a less sophisticated user. Variations exist from state to state, but by and large meet MUTCD requirements and have a mast arm with one RYG signal at the end of the arm and one that is on the side of the upright mast. Common specifications for these systems allow synchronized system operation between trailers by three common means: 1. precision time clocks which are synchronized before deployment, 2. a hardwire connection between trailers, and 3. radio communication between the trailers.
Single lane road closures often involve bridge repair or reconstruction. It is not uncommon for these projects to grow into more complex phasings because of side roads, or private driveways that need to be signalized. Sometimes the signals on these driveways are non-MUTCD compliant, having only 1 traffic signal and having the signal at less than 8 feet. This is allowed because such driveways are private, not public right of ways.
As technology has improved and advanced, portable traffic signals can reproduce most simple to complex traffic phasings, from the single lane road closure to a fully actuated dual ring quad left turn with pedestrian signals utilizing multiple portable mast arms to be used simultaneously for a single approach. The more complex systems that tend to follow existing standards, such as NEMA TS1 or 2070/ATC, give a greater ability to emulate real intersections and work well for intersections being constructed or reconstructed. These systems offer nearly full NEMA TS1 functionality via a menu driven user interface and industry standard conflict monitoring.
Gaining Perspective
All of today’s sophisticated portable traffic signals are possible because of the maturing of DC powered traffic control technology, DC powered LED technology, solar power technology, non-intrusive vehicle detection, and frequency hopping spread spectrum radios. Because of these technologies, the portable traffic signal is now the smart tool for today’s work zone traffic control.
OMJC’s design goals in our portable signal systems have always been to get emergency and planned work zone traffic control people out of harm’s way and reproduce what they do with safe, automated, actuated traffic control equipment, featuring traffic signal industry standard conflict monitoring. In so doing, labor costs can be reduced, traffic delay minimized and tort exposure minimized. And we have equipment that accomplishes these goals. But to show you how good today’s portable signals are, we really need to see where they have come from.
In the beginning, portable traffic signals had many difficulties. The signals had to be hardwired to the main control unit because there was no other effective way to build the system. The signals used industry standard incandescent lamps in order to meet ITE illumination requirements. These lamps consumed much more power than today’s LED indications. Thus, the cables were long, heavy and awkward, and must be routed through the work zone in some fashion. Power had to be supplied by diesel generated AC power.
The diesel – smelly and noisy - constantly needed fuel, lubrication and attention – no awards for labor reduction or environmental consciousness there. This system was ultimately effective, but not cheap nor easy, nor even very portable, but it did provide for all approaches to be simultaneously monitored by a single, centralized industry standard conflict monitor (CM). These industry standards (whether 170 or NEMA TS1 or a descendent) have enviable safety and tort defense records, thereby minimizing tort exposure.
The first wireless portable signals used distributed quartz redundant timers and low wattage DC tungsten lamps, with a liquid lead acid battery bank powering each unit. While this was easy and portable, it presented problems. Among them, there was no effective means of providing an actuated system – the system had to function as a fixed cycle length system only.
Then there were also early attempts to use a wireless radio connection. This usually involved licensing with the FCC, which is no longer necessary with the frequencies used today. The cables were eliminated, but at a greater cost, and the diesel remained, in spite of the battery banks on board. There was no flexibility – two actuated traffic movements were all you could get.
The Current Features Offered
The temporary traffic signal market place now offers many quickly deployed tools to help manage work zone traffic. These tools offer the ability to enable the motoring public to experience a minimum of delays in a variety of work zones. Optional equipment even allows an operator within the work zone to operate and or interrupt normal signal operation. Even pilot car control is available.
As mentioned earlier, one element that gives today’s portable traffic signal equipment viability is LED signal lighting. All systems today are battery powered because of the low power draw of the LED. This battery-powered equipment offers run times (autonomy) from mere hours to 20-40 days, depending upon the size of the battery bank. Some systems are designed for daily use and need to be recharged manually. With the addition of solar panel charging, many systems are now capable of running pretty much year around – now traffic signals can be green in more than one way! Cloudy days are compensated for by excess sizing of solar and battery systems. Battery lives are sustainable for up to 10 years by choosing quality battery systems. Long battery discharge cycles tend to help prolong battery life. Battery life is also largely affected by the frequency and depth of battery discharges. While liquid lead acid batteries are still the price leaders, the AGM battery (standard on all OMJC PTS) offers the attractive combination of being maintenance and corrosion free, unspillable even in an accident, and having an extremely low resting discharge rate.
The lower power consumption of LEDs also has enabled the industry to expand portable signal capabilities. Today it is not uncommon to see added signal heads for additional traffic phases, and a broader range of detection equipment being used. Emergency preemption is also becoming more common.
Today’s PTS systems, including OMJC systems, are robust and secure. Most offer significant wind loadings – OMJC’s is rated to 80 MPH. Some also offer security for the trailer itself – removable hitches, high security mounting hardware for solar panels, cell phone dialers, and even GPS tracking of the trailers.
The higher end systems, such as OMJC’s 2070/ATC based units, often provide integrated dedicated military quality spread spectrum radios (using IP type networking) communicating real time control and monitoring as an integral part of the control equipment design. The more complex high-end controllers provide multiple rings, barriers, I/O mapping, coordination, preemption and overlap functions, including trailing green overlaps, which can greatly improve traffic efficiency in a single lane road closure. You may also find menu driven interface functionality, Ethernet ports, Linux based software, web based user interfaces and computer emulators with download capabilities. They may offer state of the art conflict monitoring that utilizes the new programmable ITS monitor key enabling real time monitoring of each channel’s voltages and current draw, plus the other features expected in hardwired systems. These units are as safe, versatile and effective as anything controlling traffic in the USA.
Currently there is a new NEMA TS5 committee in place developing a standard for “Traffic Controllers for Portable Traffic Signal Systems”. It will be interesting to see where and what it ends up with. A deficiency in this industry is that, unlike the standards for permanent intersections, most current monitoring systems are proprietary or non-existent.