3174: Bridge Clearance

Bridge Clearance

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Title text: A lot of the highway department's budget goes to adjusting the sign whenever the moon passes directly overhead.

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Explanation

Roads passing under bridges (or other overhead structures) often have signs indicating the "clearance" or lowest height of the bridge above the road, primarily to indicate to drivers of taller vehicles the potential hazard of driving underneath. Warnings usually start with signs advising which routes to avoid starting down, so that tall vehicles can find a safe alternate path without having to reverse and obstruct traffic. There may be physical warning systems, immediately before the actual restrictive feature, to alert inattentive drivers and prevent far more troublesome damage/blockages at the pinch-point itself. In this comic, there is a bridge with one of these signs, preceded by a 'sacrificial' bar, indicating a clearance of 10 feet and 6 inches (3.20 m), well under the height (14 feet, 4.3 m) below which, in the USA, warning signage is typically required. The bar appears to be designed so that vehicles that are too tall collide with it before going under the bridge, alerting the driver that they need to stop beforehand.

On a turn-off, leading away to the right to meet the higher-level route that passes over the current one, there's a different sign which says "no bridge", allowing the relevant vehicles to turn off and meet the higher-grade route (potentially to cross straight over it and rejoin the original carriageway via the counterpart on-ramp on the other side of the bridge). It does this while indicating a clearance of (more than) 46 billion light years. This sign indicates a (default) clearance on this stretch of highway that is based upon the approximate radius of the observable universe. This is because there is no bridge or any known obstruction lower than this particularly generous 'limit'. The vertical post for this sign is shown continuing far above the upper edge of the sign and past the upper edge of the panel, implying that the sign has been equipped with a similar sacrificial bar at a ridiculous height above the ground. The tallest human-made structure at the time of publication is only 2,722 feet (829.8 m), and even far shorter vertical poles require special reinforcement to remain stable. The bar states that the clearance is 46 billion light years, which is equivalent to the approximate radius of the observable universe.

The title text references the fact that the Moon periodically passes over the road (or, from a different point of view, the road passes under the Moon). This implies that the road lies between latitudes approximately 28.5° N and S. The Moon passes over any given longitude almost daily (roughly every 24 hours but shifted by the Moon's own slower motion). Whenever the Moon passes overhead, the theoretical clearance reduces to the distance between Earth and the Moon (around 225-250,000 miles or 360-400,000 km), and any vehicle would now need to be shorter than this still very generous limit to not crash into the Moon.

Due to the transient nature of this situation, it costs a lot of money to adjust the sign on the right before and after this 'danger period'. While the sign might be digital, the implied overhead bar could not move sufficiently without exceeding the speed of light. So one pole would have to remain permanently at the greater height while a second pole was deployed temporarily at the lower height, and the taller pole would get snapped off every time the Moon, which moves about 1 km/s around the Earth, passed overhead and crashed into it. And this system apparently doesn't consider that other astronomical bodies, much farther out than the Moon, would frequently (if momentarily) be directly over any particular section of road, not to mention a large number of artificial satellites that pass over at relatively far lower altitudes.

There is also a problem of many countries having height restriction laws. For example FAA restricts a structure height to 2000 ft (610 m), so regardless of clearance or any airplanes present above, you won't normally be allowed to have something higher than that anywhere on US territory. There are also oversized load limitations; for example, there is a height limit before you would need a permit and potentially trip route assistance, with the height requiring a permit being between 13.5 and 14.5 feet depending on the state. Also, permit is unlikely to be granted for very high load, because of structural concerns of cargo load and/or weight load on the axles of the vehicle. Because trip route assistance will be required by law in most cases, the signage of arbitrary high height limits is not required, as making sure that vehicle fits will be taken care of during application for the permit.

Of course, all such versions of these 'no bridge' signs ignore the fact that vehicles cannot actually be physically large enough that such extreme clearances would be something necessary to anticipate. The problems with a vehicle, or a sign, tall enough to approach the Moon would be much worse than for an Earth-based "space elevator", which would be far shorter.

Low bridge heights, such as the 10 feet 6 inches (3.2 m) cited in the comic, are present on highways constructed before the US Interstate Highway system established 14 feet (4.3 m), later 16 feet (4.9 m), as the standard minimum clearance in the late 1950s. These low heights are most common in urban areas in the northeastern portion of the USA. Examples include Memorial (1897) and Storrow (1951) Drives in the Boston metropolitan area, where Randall lives, and the Merritt Parkway (late 1930s) in Connecticut. The minimum bridge clearance on the Merritt Parkway, when constructed, was 11 feet (3.4 m); these bridges remain, and clearances have tended to decline over time. Clearances as low as 9 feet (2.7 m) are present on Memorial Drive. Further examples of low clearances may be found with bridges built in the 19th and early 20th centuries, such as covered bridges over streams and railway bridges over secondary roads.

Transcript

[There is a road heading into the distance under a bridge, with an off-ramp/sliproad that veers off to the right. There are signs on the shoulders/verges/berms of both routes.]

[Left/forward-leading sign, on a pole leading up to support a pre-bridge 'hazard bar'. Both the edging of the sign and the overhead bar have diagonal warning stripes. The wording wraps across several lines, the initial line being underscored by a horizontal rule.]

Bridge

Clearance

10 ft 6 in

[Right-forking sign, on a pole that is seen going upwards off-frame, the sign having a similar style of edging and wording.]

No Bridge

Clearance

46 Billion+

Light-years

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Discussion

But when the moon is directly overhead they also have to edit the contents of the sign Mathmaster (talk)

The Moon being overhead only applies to places in latitudes roughly between 28.5 degrees N and S, at its absolute most extreme inclinations. So, for the contiiguous US, that potentially affects only roads in some bits of Florida and Texas.

Louisiana's most southern point is very close to that, such that the 'upper limb' of our satellite would 'overhead' an additional quarter of a degree of latitude, taking in this spot and a bit more. But that location is also an island. Port Fourchon, Louisiana, seems to be the most southerly stretch of regular (mainland) road in that state, and that's still just too far north to be affected. 78.144.255.82 23:10, 28 November 2025 (UTC)

Note how the second sign extends off the panel, presumably with a warning further up for any vehicles under clearance. That’s quite the space elevator. KelOfTheStars! (talk) 01:21, 29 November 2025 (UTC)

Also note that in latitudes where the moon could possibly be directly overhead, the sun could also be directly overhead. This would also necessitate a change to the sign 24.210.252.188 02:56, 29 November 2025 (UTC)

(Moon tilt on top of Earth tilt makes the all-seasons "Sun overhead" a narrower band than the potential "Moon-overhead", i.e. Tropic Of Capricorn to Tropic Of Cancer).

For an even greater range, and lower clearance, consider the ISS. Then there's the Starlink/etc constellation 'mesh' of orbits that deliberately stretches further out. Or indeed polar-(/near-polar-)orbits for Earth Observation (Sun-synchronous orbits, slightly off polar, typically can be directly above anything up to 82-ish° N/S, being 98° and retrograde.

And clearances of GSOs (there will be locations where sufficiently geostationary satellites are pretty much perpetual, though mere geosynchronous ones may have daily (or twice-daily, on the crux of the figure-of-8 ground track) 'exposure') are so much greater than what the LEO ones would.

A particular favourite of the Russian civil/military programmes are highly eccentric (and oblique) to service the kind of latitudes they want more loiter (slow, and far away) or passage (near, but rapid) over, often in teams of craft spread across the track to pass coverage over to another when one of them zooms on out of the desired 'sweet spot'... "Tundra Orbits", I think it is? So accounting for them might involve vastly varying heights (though usually similar, overhead to overhead, barring any ascending-/descending-track differences) over a greater-than-average spread of latitudes (but still less than pure 90°-polar would, which is potentially over everywhere at some time or another). 82.132.237.174 14:06, 29 November 2025 (UTC)

When I first read the title text I thought it was talking about the tide's effect on the height of the bridge. Barmar (talk) 03:55, 29 November 2025 (UTC)

How old is the sign that needs to he updated every day? These days they have automated signs for things like travel time to important exits, that type of system could easily be used to keep the clearance up-to-date. Barmar (talk) 03:59, 29 November 2025 (UTC)

It'll cost more to at least maintain a changable sign. Perhaps power connection (unless solar+battery is enough), probably data connection (push- or pull- reconfiguration, unless relying upon continually internally calculated via RTC and the appropriate ephemera). It might not need to be visited each day, but periodic checks are going to be more than checking it hasn't been overly pierced by buckshot (or being told it's been flattened by a carelessly driven vehicle), and other charges will apply. Especially if you're covering every few yards (even hundreds of yards) of road with individually personalised warnings for that particular stretch. 82.132.237.174 14:06, 29 November 2025 (UTC)

> clearance of 10 feet and 6 inches, which is a realistic clearance..... Anything less than 13'6" (in the US) will get hit frequently. Yes, we know some bridges that get hit frequently. --PRR (talk) 06:42, 29 November 2025 (UTC)

It says realistic, not common. Storrow Drive, which should be very familiar to Randall, has a clearance of just 10 feet. --Coconut Galaxy (talk) 07:08, 29 November 2025 (UTC)

Memorial Drive, at the so-called Harvard Bridge, right near MIT, has a clearance of 9 feet 0 inches. Large commercial traffic is not allowed on those two roads. Somehow big trucks go there regardless. MIT students sleeping in the nearby dorm, occasionally awaken to a loud noise. They phone MIT Campus Patrol, say, "Truck trap," and return to slumber. 173.188.194.233 15:10, 29 November 2025 (UTC)

The current standard for bridge heights was established in 1956, when the US Interstate Highway system came into being. The minimum height was originally 14 feet (4.3 m), it was promptly (by 1960) raised to 16 feet (4.9 m); the War Defense Department had demanded 17 feet (5.2 m). Highways built before the Interstate highway system set the standard (such as Storrow Drive, 1950) had lower clearances. The Merritt Parkway in Connecticut, a pioneering controlled-access highway built in the late 1930s, had a minimum bridge height of 11 feet (3.4 m); some of those clearances today are, or approach, 10 feet six inches (3.2 m). Surviving low-clearance bridges tend to be covered bridges over streams, and railway bridges over secondary roads, all built in the 19th and early 20th centuries. Trucks designed specifically (as they were, IIRC) to haul goods over the Interstate system will struggle on these older constructs. 2605:59C8:160:DB08:E8F0:A309:4673:6AEF 16:04, 29 November 2025 (UTC)

Plenty of bridges in the UK get Bridge Strikes from trucks (despite copious warnings), and even the occasional double-decker bus that the driver gets wrong (wrong route, perhaps on a Not In Service drive to/from the depot, and forgotten what he's driving, hopefully nobody's riding above). Almost any city (and many rural locations) will probably know at least one local railway (or canal!) bridge that has massive amounts of face-protection (painted with warning stripes, words and height details, all the round warning signs, probably a 'jangly chain-bar' roof scraper and/or photoelectric warning-sign illuminator in the last stretch before it - and still visible scraping/denting on the add-on face-protection).

Not sure if it's the lowest, vehiclewise, but for the UK I found https://www.youtube.com/watch?v=Vy9PmSRwG-k (going by the video thumbnail only) as a bridge that (non-SUV!) cars can just about use. But most people couldn't even walk or ride a bike under it, without ducking. Though at least you'd be high enough in any lorry cab to know it's a barrier to your vehicle. (Well, you'd have missed/disbelieved the signage, but basically be heading at a more obvious 'wall'.) 82.132.237.174 16:16, 29 November 2025 (UTC)

Try this one in Scotland, at 4 feet 9 inches (3.2 m). 2605:59C8:160:DB08:E8F0:A309:4673:6AEF 19:48, 29 November 2025 (UTC)

I don't know what that sign says, or how smalle Scotttish feet are, but 4`9 is slightly under 1.5m. :D --88.65.244.212 22:17, 29 November 2025 (UTC)

Depending on which Stretview shots (varied examples, seemingly, by time), you also get it as 4'6" and 1.3m warnings (if exactly that in Ft+In, which it might not be, should basically be 1m 37⅙cm, so probably was rounded down to 1.3m, even if the true value was also rounded down to get 4'6", which is better than being rounded up). The 4'9" might therefore have been a typo/thinko/printo on that version of the non-metric signage, or an inadvertant round-up from the real measurement.

It looks like the signs on the bridge itself are accurate enough (in a downward-rounding direction), at least in the various versions I've discerned that are not either blurred by the StreetView process or obscured by the vegetation trailing down from above. 78.144.255.82 22:52, 29 November 2025 (UTC)

It was 1.4 m, not 3.2. I erred. 205.175.118.102 00:23, 30 November 2025 (UTC)

Would expect that those in charge of the airspace would object to vehicles passing through. 64.114.211.61 17:57, 29 November 2025 (UTC)

Hello, If I may, I was thinking that there is an implied sacrificial bar on the ~46b light year sign, as the sign post does not stop at the sign, but continues on out of the panel. This could also add context as to why it is so expensive for the moon to cross over the road, as the highway department would need to very quickly replace the sacrificial bar with a much lower one, only to put the taller one back up a couple minutes or even seconds later. Nvidietha (talk) 19:04, 29 November 2025 (UTC)

What would happen to a 45-million light-year bar attached to a rotating surface such as earth? Given that points further from the centre would be rotating quicker, wouldn't the points near the end be at and above the speed of light? Would the act of extending it and expending earth's momentum on rotating it and spend all of the speed which allows the end to approach C? What if it materialised instead of extended? As far as i can tell, the Federal Road Body does not have the budget to break the laws of the universe. 92.40.216.156 21:25, 29 November 2025 (UTC)

Just taking Earth's rotation into account, the tip would be traveling (length of pole)×2pi per day. Assuming I didn't mess up some units with my napkin math, the tip will be going the speed of light at a length of about 25 AU, which is effectively microscopic on any scale worth measuring in light years. 209.188.63.98 05:23, 30 November 2025 (UTC)

Would the presence of stuff deeper in space like stars and asteroids not warrant a constantly-changing number as the earth rotates such that the road intersects different ones? Currently, if a car with a 10-thousand light-year thing atop following the rotation of earth would likely be constantly hitting various space things as it rotates to follow the earth. 92.40.216.156 21:17, 29 November 2025 (UTC)

My thought is that, if one maintains a maximally-strict definition of "directly overhead", the need for such changes would, in actuality, be vanishingly small. Without attempting to crunch numbers, I imagine that, in the context of deep space (and assuming that deep space is finite), a cylinder 2 m in diameter, stretching from the earth's surface, would disappear into the void and hit almost nothing. 205.175.118.102 22:29, 29 November 2025 (UTC)

Has anyone yet estimated the average height of overhead cosmic whatever above the Earth's surface? Seems like a topic that someone would have already looked into, and could be used here. I agree though, I think it'd be more likely for a 2m cross-section to hit a "ceiling" at the edge of the observable universe rather than a star or planet. That being said, there are a lot of stars and planets. R128 (talk) 16:28, 1 December 2025 (UTC)

This ignores the relativistic limits. At a height of 1/(2 pi cos(latitude)) light days, the top of your vehicle would be traveling at light speed. And that is just from the rotation of the earth. In the Boston area, that works out to about 18.3 trillion feet. --Divad27182 (talk) 11:56, 30 November 2025 (UTC)

Prior to the relativistic effects, I'm confident that the forces (tension; and, not to mention, if ever originally considered stationary, the attempt to apply lateral/angular momentum) would exceed any possible material's structural strength. I'll leave it up to someone else to work out where this theoretical maximum (thoroughly triple-bonded covalent monomolecular chain of neutronium-close but hydrogen-dense material?) might actually break down.

That aside, superluminal movement/communication could be achieved by carefully coordinated 'predictive' temporary connectivity, a whole universe-spanning disc of free-floating components that link/unlink as required to 'at that moment' (by any given frame-of-reference) the vehicle/barrier-support-pole is as actually as high as it is expected to be.

Already, we need to have had the normal 'upper limit bar' variously applied/unapplied more than 14 billion years ago, in synch with how our planet's road infrastructure and its moon are precisely to be positioned, when even the latter won't actually start to exist (to eventually cause such a headache to the former) for almost 10 billion years. So you've already got to have worked that little problem out ahead of time, and thus increasingly less prescient installations perhaps all across half the cosmos. Which is a good trick, if you can do it! 78.144.255.82 18:32, 30 November 2025 (UTC)

I added height that would require permit. But wasn't able to find at which height permit will definitely never be granted, maybe because no government actually ever made such statements, but not sure. --Trimutius (talk) 13:08, 1 December 2025 (UTC)

There's a website with videos of crashes under 11' 8" bridges: https://11foot8.com/. Dbwiddis (talk) 18:22, 1 December 2025 (UTC)
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