# Too lazy to take a hard-right.August 25, 2006 4:45 AM   Subscribe

Do the curved triangular shapes made when cars (or even pedestrians) take "shortcuts" at T-junctions (seen here near my house and here in Western Sahara, although this seems like a deliberate one) have a name? What's the mathematics behind them? The psychology?
posted by Jimbob to Grab Bag (23 answers total) 3 users marked this as a favorite

If that were a highway it would be called an on-ramp (or off-ramp depending on direction).
posted by maxpower at 5:33 AM on August 25, 2006

Best answer: Line of least resistance, perhaps? Instead of walking in one direction to the junction then in effect turning back on yourself (in your first link, walking up the screen with the intention of going right), people will naturally try to cut off the corner to reduce the distance they walk.

I recall reading something about a university (I think) where no paths were laid out in the grounds. After a few months of people wearing a groove in the grass along the optimal routes between buildings, they then put paths in. I'll try and find whether it was true, and where I read it...
posted by Chunder at 5:36 AM on August 25, 2006

Response by poster: After a few months of people wearing a groove in the grass along the optimal routes between buildings, they then put paths in.

This is exactly where my question stems from. It just feels that some settings (parks, campuses, suburbs) have paths laid out that are just wrong somehow, in terms of the optimal way people like to travel. So, after a time, you see paths worn in the grass where people have cut corners, in order to walk the path their mind wants to walk, instead of sticking to the concrete.
posted by Jimbob at 5:46 AM on August 25, 2006

Probably the most comparable curve with a well-known name is the deltoid, though your figure has one side flattened. The point of having them at junctions like this is simply to allow traffic on and off the straight road without requiring them to come to the full stop or nearly-full that a 90-degree turn would require.

You may be a little interested in the tractrix as well. It's the path your rear wheels will take as they try to follow your front wheels through a 90 degree turn. Riding a bicycle in the snow can be a good illustration.
posted by Wolfdog at 5:57 AM on August 25, 2006 [1 favorite]

The Carpenter Center at Harvard University, by LeCorbusier the renowned urbanist and designer, has these paths of consensus worn off the prescribed walkways.
posted by StickyCarpet at 5:59 AM on August 25, 2006

(...that a 90-degree turn or the sharp turn necessitated by using any straight-line "shortcut" would require. The construction is aimed at keeping the rate at which you have to move your steering wheel minimal.)
posted by Wolfdog at 6:02 AM on August 25, 2006

Best answer: The anecdote Chunder is referring to about the university paths is located in a book called Emergence. Jimbob, this book goes to great lengths to quantify the shortcuts that people, ants and sheep take.
posted by Alison at 6:02 AM on August 25, 2006

Best answer: The shortcuts you seen worn into the grass on campuses are usually straight lines, not these curved things, because people aren't worried about centripetal force pushing them off their path.
posted by Wolfdog at 6:03 AM on August 25, 2006

It reminds me of a fillet in architecture/engineering/drafting. "In mechanical engineering, a fillet is a concave easing of an interior corner of a part design."
posted by teg at 6:36 AM on August 25, 2006

The junction near your house does not seem to be a "T" junction because the southerly road does not intersect the east-west road at a 90-degree angle. If you are travelling east, and then turning south (or north and turning west), it's a minimal turn. However, if you are travelling west and turning south (or north and turning east), it would be a very difficult turn without the "shortcut" you are referring to. I would guess the shortcut is intentional on the part of the road-builder, in order to allow those making the tougher turn a bit of lattitude.
posted by MrZero at 6:51 AM on August 25, 2006

Response by poster: Actually, MrZero, I should have mentioned that one is a special case. West of the intersection is a one-way track, heading east. East of the intersection, and south of the intersection is two-way, so that might influence the volume of traffic.
posted by Jimbob at 6:57 AM on August 25, 2006

The university paths are also mentioned in How Buildings Learn: What Happens After They're Built.
posted by kirkaracha at 7:01 AM on August 25, 2006 [2 favorites]

Best answer: I think the term "desire paths" is used for these, but only when made by pedestrians. I've seen especially impressive versions of them in highly "planned" cities like Brasilia, and on lots of college campuses, as mentioned. Here's a Flickr set.
Performance and theatre people, as well as obviously landscape architects and planners, seem really taken by the term. I saw this great itinerant performance/audio tour in Singapore. There's also a play by Graeme Miller. See also Certeau's essay "Walking in the City" in Practice of Everyday Life.
posted by Mngo at 7:39 AM on August 25, 2006 [1 favorite]

The technical term for engineered lanes similar to these is channelized lanes. But that is specifically for vehicular roads and is there to provide a moving turn instead of a stopping turn. Obviously for pedestrians this is not an issue and are there for different reasons.
posted by JJ86 at 7:46 AM on August 25, 2006

Response by poster: Thanks everyone, you've given me more than enough material to keep me interested as I read up on this. Best ask.me answers ever.
posted by Jimbob at 8:47 AM on August 25, 2006

I've also heard these paths called "beelines", which I think is perfect.
posted by exceptinsects at 12:43 PM on August 25, 2006

The shortcuts you seen worn into the grass on campuses are usually straight lines, not these curved things, because people aren't worried about centripetal force pushing them off their path.

Is that true? I mean even walking most people arc instead of turning ninety degrees because it's easier. At work, the carpet on the way to the bathroom is worn in arcs.
posted by dame at 1:30 PM on August 25, 2006

I've never heard "desire paths" but I have heard "desire lines." Mostly mentioned when talking about foot traffic but I see no reason it couldn't apply to driving paths as well.
posted by jessicapierce at 2:07 PM on August 25, 2006

Doors are different than intersecting sidewalks, which is the picture I had in mind. What happens near sidewalk intersections is usually like this, to the best of my experience:

with people taking a straight line across the grass. The intent in this case is to save distance; long live Pythagoras. But carpet wear near doors looks more like this, as you say:

In this case, the intent is to avoid jerky motion. The wall prevents you from taking a straight-line shortcut at a comfortable angle, but an abrupt 90-degree turn is awkward even on foot, so the curve smooths it out. It also does save a tiny little bit of distance travelled.

You might see a little curving near the upper end of my straight-line short cut as people smooth out the transition back onto the sidewalk, but usually the sidewalk is wide enough that the curved transition happens there instead of in the grass.
posted by Wolfdog at 2:16 PM on August 25, 2006

Ist it worth saying that you can't literally take a hard right in a car?

In Microserfs, Douglas Coupland has another take on the "find your own paths" thing. Microsoft staff, according to him, believe that Bill Gates watches them walking around the "campus", notes people who take short cuts and promotes them.
posted by AmbroseChapel at 5:29 PM on August 25, 2006

I've noticed that some longer paths through fields describe a slight S shape. Do humans not like to walk a very straight line?
posted by theora55 at 9:43 PM on August 25, 2006

"Desire Lines" is also the term I am familiar with, at least with regard to pedestrians.
posted by oneirodynia at 10:07 PM on August 25, 2006

Sine-Generated Curves may be what you're looking for. These are the shapes of meandering rivers, long exposure photos of flies lazing around in the middles of rooms, flexible metal bands held between two points closer together than the length of the band, ribbon candy, and lots of other things. The linked site says they have the property of minimizing average curvature, which appears to be what Wolfdog is talking about in one of his comments.
posted by jamjam at 12:25 PM on August 27, 2006

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