Understanding Bridge Lights

By World Wide Marine Training

Did you ever wonder what the red and green lights on the underside of highway bridges mean? First of all, they mean nothing to the drivers on the road. Those lights are intended only for mariners.

The U.S. Coast Guard regulates the placement of lights and clearance gauges on bridges that cross over navigable waterways. Not every bridge has clearance gauges. Where installed, they are generally vertically mounted white boards with black painted numbers.

Clearance gauges indicate the distance between the existing waterline and the lowest point of the bridge above the channel. They are usually mounted on the right-hand side of the structure, on both approaches to the bridge.

The red and green lights on the underside of highway bridges are referred to as “obstruction lights.” All bridges, other than the “clear span” type, present an obstruction at their foundation and an overhead obstruction at some height above the navigable waterway.

Red and green bridge lights are “fixed,” which refers to the fact that they do not flash or have any period of darkness about them. They are turned on from sunset to sunrise and remain on continuously, just like your porch light at home.  In most cases, bridge lights are not shown on charts.

Bridge lights have two basic functions.  The green lights mark the center of the navigable channel, while the red lights mark the channel’s port and starboard limits, as well as any obstructions (columns, abutments, etc.). This system is easy to remember, since on land we use red for stop and green for go.

The green lights are positioned under the center of the bridge as a “range.” A range is a pair of lights that is intended for observation in a vertical line. That is, one directly above the other. As a mariner approaches a bridge, if the green lights are observed one directly above the other, the vessel is in the center of the navigable channel.

Being in the center is an especially important issue for a deep draft vessel passing under a wide span bridge in a narrow channel. In this scenario, it is possible to go aground while under the bridge. Many mariners assume that the navigable channel occupies the full space between bridge columns, which is not always the case. It is best to remain in the center, unless oncoming traffic makes it impossible.

As for the red lights, they mark the limits of the channel and any obstructions at the waterline. There will be at least one red light to both port and starboard. Sometimes they will be on the underside of the bridge, and at other times on the abutments or fenders (the wooden structures around the bridge supports).

If the navigable channel is narrower than the opening between bridge supports, red lights will mark both the channel limits and the bridge supports. This is the case where a deep draft vessel can go aground while passing under a bridge.

The last piece of the puzzle is lighting the moveable portion of a drawbridge. This is done with one or more red lights when the bridge is down and one or more green lights when the bridge is up. The number and location of the red and green lights varies with the configuration of the bridge.

Until next time, we wish you clear skies, fair winds and calm seas!


World Wide Marine Training, LLC, is a U.S. Coast Guard Approved facility authorized to give examinations for captain’s licenses up to Master 200 Tons, Able Seaman up to Unlimited, STCW Basic Training, Radar, ARPA and other Endorsements. Please visit www.worldwidemarinetraining.com or call toll-free 866-249-2135.