Editing 3174: Bridge Clearance

{{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==
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. In extreme cases warnings usually start with signs advising which routes to avoid starting down, but on most highways the signs are placed in the immediate vicinity of the offending structure. The comic shows one such sign with the ostensibly low bridge in the background. An exit ramp, which conveniently avoids the bridge, has another sign has its own clearance information that, while technically accurate, is humorously pointless in the XKCD style as it is unlikely that a road vehicle on Earth will care about running into a star in a galaxy far far away.  

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 {{w|Orbit of the Moon#Inclination|latitudes approximately 28.5° 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 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 the same pedantic highway department would likely feel inclined to dynamically update the sign to remain technically correct at all times. This would be theoretically practical with active signage and some basic programming.

==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
{{comic discussion}}<noinclude>

[[Category:Engineering]]
[[Category:Space]]