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| Bridge Clearance |
 Title text: 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. 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.
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 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 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 latitudes approximately 28.5 degrees 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 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 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 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 will be passing 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 merely 2000 ft (610 m) so, regardless of clearance or any airplanes present above, you won't normally be allowed to have something that high above the ground (at the point just before it becomes considered a 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 (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 are most common in urban areas in the northeastern portion of the USA. Examples include Memorial (1897) and Storrow (1951) Drives in the Boston MA metropolitan area, where Randall lives, and the Merritt Parkway in Connecticut (late 1930s). 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 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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