The Moonlight Towers
While traveling along the freeways you may have noticed that some lights are mounted on huge towers rather than short poles. The idea of lighting a large area with high-mounted lights dates from the dawn of the electrical era, with carbon arc lights. Here’s one from San Jose.
And here is one from Minneapolis, in Bridge Square in 1883.
Later ones were often termed moonlight towers. In Austin, Texas, 15 of the original 33 remain, and in 1976 they were listed on the National Register of Historic Places.
The First Modern Tower Lights
Modern tower (sometimes called high-mast) lights for freeways were first used in Minnesota in 1971, at Worthington. In their modern form they use three to six luminaires on 100- to 120-foot monopoles, with the luminaires (engineer-speak for the actual light fixture part) capable of being lowered for servicing by a pulley system powered by an external motor. Here you can see how it works.
These poles look to be conventional painted steel, but soon manufacturers switched to self-weathering steel. The initial surface corrosion on these forms a protective coating that prevents future corrosion, hence their reddish brown color as opposed to the bare stainless steel of the lower-mounted freeway lights.
Other early installations were the Fish Lake interchange (what is now U.S. 169, Minnesota State Highway 62 and U.S. 212), the Valleyfair parking lot, Burnsville Mall and the airport access road. One of the poles in the U.S. 169 installation for several years held a raptor nest. When the lights were removed in the 1990s, dismantling that pole was delayed until after nesting season. As a local newspaper put it, “End of nesting season means it’s lights out for the raptors.”
Over time these older 1,000-watt mercury vapor installations were retrofitted to 1,000-watt metal halide lamps, which vastly over-lit the area. Metal halide lamps are more or less just mercury vapor lamps with metal halide compounds added to improve efficiency and light quality at the cost of service life; the older of two types of metal halide can (generally) use the same fixtures as mercury vapor. Eventually these started to be retrofitted to high-pressure sodium (HPS) lamps, which required new luminaires. Given that HPS is about twice as efficient as mercury vapor, officials often would replace every other luminaire, disconnecting the others in place.
Of these early installations only the one in the Valleyfair parking lot remains, with the metal halide lamps running in the original mercury ballasts. Bulb-geeks will notice that they’re using enclosed-only rated metal halide lamps in an open fixture. Metal halide lamps can fail violently due to the corrosive nature of the halogen additives to the arc tube, and using them in open fixtures requires special lamps with an internal shield. Here’s hoping no one is standing beneath them if one explodes.
Recent Tower Lights
Probably the first HPS installation was Minnesota State Highways 13 and 77, in the late 1970s, followed by Interstate 494 and Minnesota State Highway 5, in the early 1980s. Due to the higher light output per lamp, high-pressure sodium installations typically used fewer luminaires: three on 100-foot poles, four or occasionally six on 120-foot poles, as opposed to six or eight. The 1990s saw an explosion in their use, on portions of I-494, I-35W, I-94 and U.S. 169 at MN 13.
The LED Conversion
For a time driving along our freeways, you could see the cool moonlight glow of the low-level LEDs contrasted with the golden yellow of HPS towers. It took longer for LED products for high-mast lights to hit the market, due to needing Minnesota Department of Transportation (MnDOT) approval, but now MnDOT has two approved products for high-mast LEDs: the Cooper Industries “CST Celesteon” and Holophane “HMAO LED III.” Unlike the conversion of the lower lights, where the contractor used products from both American Electric and Cooper (and products from Philips are common from installations before the mass conversion, especially in the northeast metro), all high-mast installations I’ve seen are only the Holophane product.
HPS draws 1,100 watts when you count the ballast. The LED fixtures use 319 watts for the asymmetrical and 627 watts for the symmetrical, depending on the beam pattern. Like other LEDs they can be ordered in 3000K, 4000K and 5000K color temperatures. MnDOT has selected 4000K (like a cool, white fluorescent lamp) for all lighting. Although 3000K might be appropriate on city streets, cooler color temperatures have their advantages. They’re more efficient, provide a stark contrast between the ambient highway light and red and yellow traffic signals and vehicular lights, and the side effect of preventing people from falling asleep is an asset on our freeways. They come with drivers capable of two-way communication for fault reporting and dimming, capabilities currently not utilized.
Another Use for Tower Luminaires
In the past, MnDOT used “turnpike” or “offset” lights that could reach farther. These lit wider freeways (five lanes or more) than traditional lights since they were mounted at a 45-degree angle. In the late 1990s and early 2000s, a trend developed to use these for even narrow freeways, using the increased reach to mount the poles farther back to reduce the number knocked down by motorists. Despite their efficiency, the luminaires were derided for years by people concerned about light pollution. MnDOT decided to quit installing these; then it removed all existing turnpike-style lights during the LED conversion.
LEDs have better optics, but that works only so far. To compensate for not mounting them at an angle, much more powerful luminaires were installed (which simply bounce the extra light off the snow into the sky for half the year). For narrow freeways it was found adequate to replace the smaller of the two sizes of HPS lamps with the larger of the two sizes of LED fixtures.
Something more was needed for five-lane freeways, where the larger-size HPS lamps initially were used. On an ad-hoc basis an LED floodlight fixture was mounted (facing down) on parts of the I-35W and I-94 commons. But enter the idea of mounting standard tower luminaires on the short poles. These ones use 376 watts. Not much of a savings compared with the 400-watt angle-mounted HPS lamps they replace. If mounted at an angle, fixtures using half that wattage likely could have sufficed.
Future LED Conversions
With the tower lights, the conversion of MnDOT fixtures to LED is nearly complete. All that’s left are some of the tunnels. Initially the I-94 tunnel was going to be left as HPS indefinitely, but truck drivers illegally using the tunnel during a construction project destroyed a number of the special fixtures. The special light-pipe fixtures were discontinued years ago, and with MnDOT unable to get more of them, there’s no choice now but to convert it to LED. MnDOT will do so in a future project.