3174: Bridge Clearance

2025-11-29T12:54:25Z

2001:16B8:CC06:E000:2758:EDCA:5D03:239B: /* Explanation */ A light 7.5 t trailer that I'm allowed to drive with my car license is already typically 3.20m high.

{{comic
| number = 3174
| date = November 28, 2025
| title = Bridge Clearance
| image = bridge_clearance_2x.png
| imagesize = 450x233px
| noexpand = true
| titletext = A lot of the highway department's budget goes to adjusting the sign whenever the moon passes directly overhead.
}}

==Explanation==
{{incomplete|This page was created BY A BRIDGE WITH NO CLEARANCE. Don't remove this notice too soon.}}
Bridges will often have signs indicating the height of the bridge, primarily to indicate to drivers of taller vehicles the potential hazard of driving under them, sometimes along with {{w|Bridge strike#Passive devices|physical warning systems}} to alert unobservant drivers any imminent errors. In this comic, there is a bridge with one of these signs and 'sacrificial' bars, indicating a clearance of 10 feet and 6 inches (3.20m) , which is a pretty shallow clearance for a highway bridge. 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", itself 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 title text references the fact that periodically, the Moon passes over the road (or, from a different point of view, the road passes under the Moon). This implies that the road lies between {{w|Orbit of the Moon#Inclination|latitudes approximately 28.5 degrees N and S}}. The moon passes over any given longitude almost daily ({{w|Sidereal time|roughly every 24 hours}} but shifted by the Moon's own {{w|Lunar month#Sidereal_month|slower motion}}). Whenever this all lines up, the theoretical clearance reduces to the distance between Earth and the Moon (around 225-250 thousand miles), and any vehicle would now need to be shorter than this ''still'' very generous limit, in order to not crash into the Moon.

Due to the transient nature of this situation, it apparently costs a lot of money to keep accurately adjusting/readjusting the posted advisories before and after this 'danger period'. And this system apparently doesn't take into consideration that other astronomical bodies, much farther out than the Moon, would frequently (if momentarily) be directly over any particular section of road.

There is also a problem of many countries having {{w|Height restriction laws}}. For example FAA restricts height to merely 2000 ft (610 m), so regardless of clearance or any airplanes present above you won't be allowed to have a ground vehicle higher than that anywhere on US territory.

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 tall enough to approach the Moon would be ''much'' worse than for an Earth-based "{{w|space elevator}}", which would be far shorter.

==Transcript==
{{incomplete transcript|Don't remove this notice too soon.}}
:[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
{{comic discussion}}<noinclude>

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3174: Bridge Clearance