Talk:3174: Bridge Clearance

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)

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)