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Median Mount Lighting
Cost saving alternative for highway retrofits

The availability of federal dollars for highway improvements is spurring a massive retrofit of highway lighting systems across the nation. Much attention is being focused on cobrahead systems, many of which have been in place since the mid 1960s. The objective is to curb operating costs while improving driver visibility.

One option being selected for many retrofits is median mount lighting systems, which can be installed on existing cobrahead poles at a ratio of one median mount fixture for every two cobraheads. Besides a reduction in installation costs, a major benefit is 50 percent less maintenance.

To clean or replace cobrahead, units, maintenance crews usually have to close a single lane of traffic on one side of a four-lane highway for as long as required to service all fixtures on that side. The lane might be closed for several miles, at a cost ranging from $500 to $5000, depending on traffic volume and location. Once the fixtures on one side of the highway are serviced, crews will then shut down a single lane on the other side to make necessary repairs.

In contrast, the median mount fixtures require that only one lane be closed, with all repairs made from that lane. The time required is cut in half, making the cost for shutting down the road 50% less.
Improved visibilityDuring the retrofit in most instances, one 400-watt high pressure sodium median mount fixture is used to replace two 750-watt mercury vapor cobrahead units. Not only are energy costs reduced, but the quality of the light is improved, with a brightness to darkness ratio of 4.3:1 for the median mount fixtures compared to 14.2:1 for the cobrahead units . The lower the ratio, the better the visibility. What results from the higher cobrahead ratio is constant change between light and dark areas, which requires the eyes to continually adapt. This can lead to increased driver fatigue.


 Before - Cobra-heads After- Probeam
Lamp 2 - 400W HPS Lamp 1 - 750W HPS
Mounting 1.83m (6') arms Mounting Shepherd's Crook
  13m (42') above pavement   13m (42') above pavement
  79m (260') on center   79m (260') on center

Results Cobra-heads Probeam Savings
Avg. footcandles 1.0 1.0  
Min. footcandles 0.18 0.40  
Avg.:min. uniformity 5.6:1 2.5:1  
Max.:min. uniformity 14.2:1 4.3:1 9.5 KW
Connected load per mile 18.6 KW 9.1 KW  

.

Reduced operating costsIllustraustration above shows the advantages of the median mount fixtures vs. the cobraheads. The cobrahead application includes two 400-watt mercury vapor fixtures, with six-foot arms, mounted 42 feet above the pavement, 260 feet on center. In the “after” application, one 400 watt high pressure sodium median mount unit, with a shepherd’s crook arm, is installed 42 feet above the pavement, 260 feet on center. Note that the average footcandle level remains the same for both fixtures, with higher minimum levels for the median mount fixtures.Because the minimum footcandle value is increased by more than two times, the uniformity of light is significantly improved. The maximum light level produced by the median mount system is 33% less than the cobrahead system, showing that a cycle of bright spots and dark spots will be eliminated because of the uniformity of illumination provided by the median mount system.
While the initial investment for a median mount lighting system may average 25% more, as you can see, the cost can be justified by the savings incurred in operating and maintenance costs each year. The typical payback period for retrofitting a cobrahead system with median mount fixtures is about one year.
Fixture selection guidelinesMedian mount lighting fixtures are as applicable for new construction as they are for retrofits. With approximately $18 billion available for new highway construction in the coming year as part of the federal budget, lighting is an area likely to be targeted.

When selecting lighting fixtures for highway applications, consider these points: 1. Make sure the fixtures are UL listed for 40°C. This is to help assure the safety of maintenance personnel. Having a luminaire designed to operate properly in a 40°C ( 104°F) ambient condition will mean the electrical components will last about twice as long as a luminaire designed for the typical 25°C (77°F) ambient condition.

2. Examine the fixtures’ lighting performance in terms of uniform distribution and glare control. A lighting system that produces uniform light levels on the entire roadway will reduce eye fatigue since the eyes aren’t in a constant state of adaptation from bright spots to dark spots. Uniformity also makes the lighting more pleasing to the general public. Proper lighting distribution will maximize efficiency, minimizing energy consumption, maintenance expense, and pole knock-downs. Numerous research studies have shown that glare has little effect on visibility, but a system that minimizes glare will likely gain the approval of the motorist more quickly than other factors.3. Consider the energy efficiency of the ballasts, with a goal to achieve less watts per footcandle. It is incumbent on the lighting designer to select luminaires that not only meet the requirements for photometric performance but also consume the least amount of energy to produce the required results. When analyzing electric performance data, it is important to compare the total input watts to the lamp watts. Some ballasts have relatively low input watts and the designer is left to assume that the lamp watts will equal the rated lamp watts, say 400 watts for example, implying low watts loss for the luminaire. Unfortunately, many of those “low loss” ballasts operate the lamp at 20 to 30 watts less than the rated lamp wattage, at 370-380 watts, for example. Not only will the resulting light levels be 5-10% lower than required, but the actual watts loss is the difference between the actual lamp wattage and the input wattage (20-30 watts higher than implied).4. Ask that the manufacturer’s sales representative perform a computer analysis, which should show enough footcandle values on the roadway to paint an accurate picture of the lighting system’s performance. A higher number of calculation points for a given area will generally provide a more accurate model of real performance. The designer should require a
grid of calculation points that has two lines per lane: and at least ten rows between two adjacent poles.By stepping away from the “cookie-cutter” approach to roadway lighting design, the lighting designer can specify a lighting system that provides excellent visibility, reduces energy consumption by at least 50%, and reduces maintenance expense by about 50 percent. The median mount roadway lighting system is one way to achieve these lofty goals.


 
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