Why are the US 6 tunnel lights west of Golden only over half of the roadway?

Jayson Luber | Denver7 Traffic

By: Jayson Luber

JEFFERSON COUNTY, Colo. — Ron from Jefferson County writes, “Along Clear Creek canyon, US 6, as you enter any of the tunnels the lights are on halfway and then going the other way, they're on halfway. Why don't they just go down center?”

There are many factors when it comes to lighting any roadway tunnel — tunnel length, number of lanes, direction of travel, sloped approach, divided roadway, daytime and nighttime conditions, among others. To add to the complexity of any tunnel lighting, the length of a tunnel matters. Long tunnels are divided into zones, each of which requires a different design and system specification. In other words, a long tunnel like the mile and a half Eisenhower and Johnson tunnels along Interstate 70 at the Continental Divide is much more challenging to light adequately than the shorter 500 to 600 foot long tunnels along US 6 in Clear Creek Canyon.

There are three tunnels on US 6 between Golden and Highway 119. There are two more between Highway 119 and I-70. When you enter a tunnel on Highway 6 in Clear Creek Canyon from either direction you will see a row of LED lights above that lane. They are spaced about 10 feet apart and seem from my naked eye to provide lots of light when entering the tunnel. Then the lights abruptly end at about the half-way point in the tunnel.

According to the Federal Highway Administration Lighting Handbook, the most significant factor in determining how a short tunnel should be lighted is how big the exit portal is in an approaching driver’s view. Additional factors are the presence of pedestrians or cyclists, whether the tunnel is undivided with bi-directional traffic, and traffic speed and volume.

Engineers with the Colorado Department of Transportation (CDOT) told me the lighting isn't continuous throughout these tunnels as they are designed to provide a smooth light transition as you enter. According to the Lighting Handbook, for short tunnels, the driver adjustment to the tunnel lighting can be complicated by the appearance of a bright exit portal. The brightness of the exit portal can reduce or prevent the driver’s eyes from adjusting optimally to the light past the exit of the tunnel. I’m told the halfway lighting design of the US 6 tunnels is based on this, as well as studies showing that it requires less lighting while still ensuring a safe and comfortable experience entering and exiting the tunnel. Additionally, daylight is considered to provide adequate illumination for a distance of roughly 50 feet from the exit portal, thus, not needing additional illumination.

The Federal Highway Adminsitration said most short tunnels are unlikely to have supplemental daytime lighting through the entire tunnel. More commonly, a short tunnel will be lit along just a part of its length. Possible exceptions to this are short tunnels that have frequent pedestrian or cyclist presence, or those with curvature so the exit portal is not in the view of approaching drivers. Those exceptions don’t come into play with the Clear Creek Canyon tunnels.

One of the tricky aspects of lighting any tunnel is matching the outside light intensity with LED lights inside a tunnel. These tunnels along Highway 6 use LED bulbs to make them more energy efficient and reduce electricity costs, but they are also connected to a photocells lighting system.

The photocells are strategically placed so they will automatically adjust the brightness of the LED bulbs inside the tunnel based on the amount of natural light outside. Traffic engineers said the photocell system ensures the best visibility for drivers at all times of the day, from bright sunlight to overcast conditions to night time. It is similar to the system inside the new I-70 Central tunnel near York Street that I covered just before one of the tunnels opened to traffic in 2021.

As for the actual lights inside all five Clear Creek Canyon tunnels, they were replaced between April 2021 and April 2022. The $5 million project included new LED lights, new lane traffic control signals at the tunnel portals, and other infrastructure updates.

Denver7 Traffic Expert Jayson Luber says he has been covering Denver-metro traffic since Ben-Hur was driving a chariot. (We believe the actual number is over 25 years.) He's obsessed with letting viewers know what's happening on their drive and the best way to avoid the problems that spring up. Follow him on Facebook, Twitter or Instagram or listen to his award winning Driving You Crazy podcast on any podcast app including iTunes, iHeartRadio, Spotify, Podbean, or YouTube.