Our thanks to Sean Welsh for this excellent, detailed explanation of bridge clearance boards.

Bridge Clearances Explained
by Sean Welsh USCG licensed Master inland, Mate near coastal
I was asked by the moderation team to write a few words about bridge clearance markings, after the comments
on a recent post revealed that there is a lot of confusion on the subject. That seemed straightforward, but, as it
turns out, it’s messy. That probably accounts for some of the confusion, so let me see if I can shed some light.

There is a standard for these markings, which are officially known as “Vertical Clearance Gauges” on the ICW
and other federal waterways, codified in federal law as 33 CFR § 118.160. Among other things, this standard
requires black markings on a white background, “foot marks” (horizontal lines of a specific size), specific spacing
of numbers, location at the right side of the channel, and regular maintenance.

This Vertical Clearance Gauge complies with the regulations. “Foot marks” every foot show the clearance; on this board the numerals are next to the foot marks every other foot. This board reads just a hair over 65′, as the entire foot mark and a little white below it is visible.

If all bridges were marked according to this standard this would be an easy post to write. But the reality is that state or county road maintenance departments, railroads, and other bridge owners, and not the federal government, are actually responsible for installing markings on bridges, and there appears to be little consequence for these entities when they break federal law by straying from the standard. The bridge that prompted this post, the infamous Wilkerson Bridge in North Carolina, is one of the many non-compliant bridges and thus the source of much confusion.

My own photo. This board, not compliant with standards. This board reads a couple of inches more than 64′. It’s hard to read because non-compliant colors have been used and the agency has failed to “… maintain each gauge in good repair and legible condition.”

Apart from the markings being a noncompliant white on a green background, the gauges on this bridge also lack
foot marks (horizontal lines) of any kind, and have numbers every foot. Since the numerals are 12" tall, this
means each number abuts the numbers above and below it. This is by no means the only bridge with this type of
marking, but it gets the most discussion here because the bridge is lower than ICW standards and is on a non-
tidal stretch of waterway, making it a tight fit for many sailboats.

On this type of clearance gauge, the clearance to low steel is between the largest number that is completely visible (not covered by any water at all) and the next larger number that is wholly or partially under water. This is a critical point to remember, because the human brain wants to see the largest number that is “mostly” visible, and count upwards from there. Among other things, this is because we are conditioned to see numbers  increase from lower to higher elevations, and on a bridge gauge it’s the other way around.

This stands in contrast to many standards-compliant gauges, where the numerals are often (but not always) next to, rather than above, the foot marks. When you see a gauge where the foot marks fall mid-numeral, it is the bottom edge of the mark, and not the bottom edge of the numeral itself, that shows the level.

On the ICW, most compliant numerals will be either 12 or 18 inches tall and spaced every two feet. But on bridges sized for ships or large tows, which need to be visible from much further away, you will find gauges where
numerals may be spaced five or ten feet apart and are much larger in size. Compliant gauges, whether on fixed or movable bridges, show the clearance to “low steel” which is the lowest hard part of the bridge that is directly over the navigation channel. The edges of the channel are defined by the  red channel limit lights, which are often on the fenders but sometimes hang below the span. If you stray outside these markers you may have lower clearance. Also, the “range lights” which are the green mid-channel lights that hang below the span, are not considered “low steel” and thus may hang down lower than the clearance measurement. If your clearance is close, pass between the range and the channel limits and do not take the range down the middle.

Having said that, once again there are some agencies out there installing non-compliant gauges and I have seen
some, notably on drawbridges, which show the clearance “at center” as noted on the sign. When in doubt, call
the bridge tender and ask. They are generally not allowed to tell you the clearance, but they can usually state
whether the gauge reads low steel or center-span, and in the case of low steel gauges, how much additional
clearance may be available center span.

While I am on the subject, now is a good time to remind everyone that bridge clearances marked on charts show
the low steel clearance at “mean high water” (MHW) for tidal bridges, and at “normal pool” for river bridges.
Actual clearance may be less; spring tides can be well above MHW and there may be freshet on top of that, and
river pools vary greatly.

Bridges, their clearances, and movable bridge regulations and schedules can be found in the relevant edition of
the Coast Pilot. Every skipper should carry the local Coast Pilot edition on board, available as a free download.
Temporary clearance changes, for example due to construction work, can be found in the current issue of the
Local Notices to Mariners (LNMs), which, again, every skipper should download and check as part of their
voyage planning.