2728: Lane Change Highway

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Lane Change Highway
I just think lane markers should follow the local magnetic declination.
Title text: I just think lane markers should follow the local magnetic declination.


Highways are large roads designated for high-speed traffic. Like in 253: Highway Engineer Pranks, Randall proposes an ineffective highway design, which reportedly got him fired.

Highways normally have several, but not a fixed number of lanes; more lanes may be included on parts of the highway with higher traffic flow, and the design decision can interact with entrances and exits to the highway. One common structure is to merge a lane from the right to the left after an entrance, to remove the extra lane created by the merge of the entry ramp. Drivers are expected to merge into one of the lanes further left before the lane on the right ends.

Here, Randall has designed a highway where the lanes are not aligned to the road, thus constantly expanding to the left and requiring merges from the right. This is highly impractical, as each individual lane merge is a difficult maneuver compared to normal driving; being forced to perform these near-constantly is a large hindrance at the least and a large danger at the most.

Effectively every car would have to drive with their left turn blinker on constantly in order to drive in a regular straight line. Alternatively cars could stage their lane changes which would make them swerve gently back and forth across the road. Since everyone will choose a different strategy, the road would be chaos. People would almost never try to make a right lane change since lanes to the right end sooner.

While considerably less extreme, a section of U.S. 101 "North" (which is actually traveling west) in southern California is somewhat like this. The road is four lanes when crossing over I-405, with an additional lane immediately entering on the right from I-405 South, but three of these lanes become exit only lanes in the San Fernando Valley (at Haskell Avenue, Havenhurst Avenue, and Topanga Canyon Blvd) and two in Ventura County (one in Thousand Oaks and one in Ventura), so all through traffic must merge into the two lanes that enter on the left carrying traffic from I-405 North.

The title text refers to local magnetic declination, the angle between the magnetic north pole and the true (geographic) north pole. This is almost always not aligned with road directions, so lanes following it would slant across the road as in this comic. In addition, magnetic declination varies over time due to polar wandering, so the lane markers on the highway would need to be regularly updated.


[In a white panel a black road starts at the bottom left and curves gently towards the right until the middle of the panel, where it begins curving slightly to the left ending in the top right corner. The road has space for four lanes divided by lane markers, but the markers does not follow the direction of the road but goes more from left to right than the road, even though they do follow the general curve of the road, but never the actual direction of the road. This results in all these lanes to end near the right edge of the road. Every time the lane gets too near the edge the lane lines stop and on the end of the lane there is a curving arrow pointing left. Below the arrows the word "Merge" is written. This happens seven times on this small patch of highway. At the bottom and top there are the lane marker lines entering and exiting, so there are always four lanes. After the three starting at the bottom, seven new ones begin, the last just below the top, where the final merge arrow also is, stopping the last of the right lines from exiting at the top.]
[Caption below the panel:]
I got fired from my traffic engineering job for building a highway where you could only travel by lane changes.


  • A faint grey outline is visible under the caption, as if text was rewritten over a semitransparent layer, which was never deleted.

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There's a section of the M25 motorway around London which does this... Never did like it. 07:14, 24 January 2023 (UTC)

I hope you are kidding ;-) Although there are some funny histories about that road. For instance Good Omens by Neil Gaiman and Terry Pratchett. (Now a series - see Crowley Creates (and Destroys) The M25 - Good Omens. :-) --Kynde (talk) 08:25, 24 January 2023 (UTC)
Not quite the same thing, but in my city there are many roads that run through a substantial portion of town, occasionally varying the number of lanes by either adding a new one immediately following an intersection, or converting an existing lane to turn-only at an intersection, and then after the intersection only having as many lanes as were permitted to travel straight through the intersection. But for one or two of these roads, in order to drive the full length of it, you will have to make several lane changes in the same direction (I can't recall whether it's to the right or to the left), because through one intersection where only a single lane is able to go straight, that same lane eventually ends up becoming a turn-only lane at another intersection somewhere farther down the road. It's obviously not like Randall's example, but it is a mild annoyance when you end up driving that stretch of road often enough to notice it. Dansiman (talk) 23:18, 24 January 2023 (UTC)
A lot of highways in France do something similar. At every ascend, the ascending traffic gets its own new lane, presumably to keep ascending cars from doing merging manoeuvres. To keep the same number of lanes, the leftmost lane merges into the adjacent lane before the ascend. So if you simply stay on your lane, you kind of drift to the left with every ascend. I am not sure if this really helps to cut accidents, but I think it is a clever solution at least for some accident-prone ascends. 08:32, 24 January 2023 (UTC)
That sounds like using passing lanes, so that slower traffic, with more difficulty going up the hill, like transport trucks, will use the outside lane and faster cars will be on the inner lane, to not be significantly impeded by the slower traffic. Nutster (talk) 14:44, 24 January 2023 (UTC)
That sounds odd to me. Even taking into account that my "left lane" is the US/French "right lane" (the one close to the kerb, furthest from the median/dividing line), "outside lane" to me means the overtaking or 'fast' lane, with "inside lane" the one most to the roadside (the driver is on the outside edge of a vehicle, assuming the usual local parity between Foo-side-driving and Bar-side-driver).
But I do know some roads (on undulating landscapes) that on the bottom of an uphill section will have a new road-edge lane (vehicles expecting to go slow will filter into that, hopefully without crawling past a marginally slower climbing vehicle to get ahead of them) and then, near the top of the rise, the centre-lane is forced back into the left just in time to encounter the end of the contrary direction's two-lane rise up its hill.
That is, three lanes for most of the time, the centre being assigned to the direction going uphill on any given stretch (a permant type of 'Tidal Flow' traffic management system). 19:04, 24 January 2023 (UTC)

Saved the image to the wayback machine here https://web.archive.org/web/20230124073752/https://xkcd.com/ Mushrooms (talk) 07:41, 24 January 2023 (UTC)

Doesn't that happen automatically? I like they are there, but do webcrawlers not manage that on a daily basis? --Kynde (talk) 08:17, 24 January 2023 (UTC)
Huh, didn't know that. Better safe than sorry! Mushrooms (talk) 08:33, 24 January 2023 (UTC)
Indeed, it doesn't hurt to do so. In fact, xkcd is actually archived every day, multiple times. I've also configured my bot to archive xkcd and explainxkcd when a new comic comes out automatically through archive.org. —theusaf (talk) 21:35, 24 January 2023 (UTC)

It's really a very wide single-lane road. The left lanes originate from the edge of the road so no cars feed into them, and on the right side once you merge there is no where to go except to merge into the next right-hand side, so the net effect is that the road is 4 lanes wide, but is functioning as a single-lane road. That assumes everyone is entering from the right side. But I guess they could be entering from the left but still in a very short time all cars are on the right. Rtanenbaum (talk) 12:24, 24 January 2023 (UTC)

This is very similar to major roundabouts, in the UK at least, that have a spiral outwards. If entering a four-way junction (and there can be more feed-offs than that) , you may be invited to assume one of three entry lanes (as soon as the feed-in is wide enough to accommodate them) for left (the side of the road upon which we drive), forward or right that lead onto one of three lanes going clockwise round the island.
(Sometimes the left-lane becomes marking- or even curb-separated from the island lanes, effectively skirting the island so there's no waiting for traffic on the island to pass. It merges with the feed-out lane, or becomes a two-lane carriageway direction, some useful distance from the junction.)
On safely entering the junction (by giving way to anything already on it or being filtered by traffic signals), the semi-perpendicular lane markings (oblique crosses at the lane-edge intersections) guide you to the outer/inner or any median encircling lane which, as each outlet is passed, shifts over by one with a new 'inner' lane for that latest input road's "(almost) all the way round" traffic. (For some junctions, 180 degree change of travel is also a necessity, e.g. due to no cross-traffic (right-hand turning) possible on the lead up to the island, but a sub-junction comes off it in that direction anyway.)
It tends to work best on the rounder roundabouts with periodic entry/exit points, or on the truly huge ones that act like a town-centre one-way inner ring-road, only without the town. When there's a large complex with straights and curves, the guiding lines (especially across the 'crosses' might not be so obvious (if they are for the first car on the road, the one immediately behind might not have sufficient sight of the outward jinks in the indicated path and lose track of which path they should be on (especially if it is their first use of the junction) and the cross-overs can get worn and/or dirtied to make it less obvious), so inevitably there's lane-drift (and more wear/obscuration of the lane-guides).
But, in general, with only accidental merging needed ("no, not this exit, it's the next one... there's nothing behind, so quickly...") and continuous lane-generation (to which the rarer all-the-way-round traffic can shift over into), I think there's a parallel. But this not being the inspiration or reference. 13:12, 24 January 2023 (UTC)

I feel like this comic was posted later in the day than usual. It would be interesting to see a graph of what time of day comics were posted in (ignoring the day of posting, just in hours since the previous midnight EST) 14:41, 24 January 2023 (UTC)Bumpf

Yes, it was very late, after midnight EST. Barmar (talk) 14:56, 24 January 2023 (UTC)

I personally would LOVE this road, as I would stay to the left and floor it. SDSpivey (talk) 15:30, 24 January 2023 (UTC)

Dang, I must have missed Randall visiting Cracow. Swerving lanes are the landmark here. One example: [1] Follow the road to find another. -- 09:22, 25 January 2023 (UTC)

All I'm seeing at that link are turning lanes that branch off the main roadway to allow cars to exit. The main two lanes seem to continue straight through, until eventually they merge into one. Jkshapiro (talk) 22:12, 19 November 2023 (UTC)

Someone who knows what a magnetic declination is should add that to the explanation 17:47, 25 January 2023 (UTC)

Is it normal for the text under the image to have weird shadows like this text has? anonymous(talk) (talk)