a signage mystery

[cellio]

I-376, like many other highways, has those overhead digital signs that somebody updates with topical messages like "accident, right lane closed 1 mi" or "stadium parking exit 72A" or, when they've got nothing better to say, "buckle up -- it's the law". There are two of these signs on my commute that, in their default states, say "distance to downtown: N mi, M min". Which, while usually not especially helpful to me (I live five miles from downtown), is still more useful to me than seatbelt nags. (I always use seatbelts.)

This morning, while stopped in traffic near Oakland, I saw one of those signs update from "4 mi, 5 min" to "4 mi, 6 min". That was less inaccurate, but far from accurate -- I reached downtown about 25 minutes later. (This is all very unusual; two of three lanes were closed due to a bad accident. My commute is sometimes slow, but I don't remember the last time I was in stopped morning traffic.)

It got me wondering -- do the indicators on those signs update automatically based on sensor data or are they human-controlled? The fact that an update happened but didn't jump to a more-appropriate number makes me think that we're dealing with an automated system that only bumps one unit at a time. (I would hope that a human would have updated it to warn about the accident.)

Why would it be designed to only increment in single units? Or is it a bug? What are the inputs to these signs, anyway?

Comments

[hudebnik]

I can confirm this: cell phone location traces are one of the more valuable signals we use. If a lot of cell phones on a particular stretch of road are moving slowly west, but rapidly east, it indicates a traffic jam on the westbound side; if there are almost NO cell phones on a particular stretch of road that normally has lots of them, it indicates a road closure; etc.

Cell phone location traces are also used to convert addresses to locations. For example, an indoor shopping mall typically has one street address, but lots of entrances, some of which are loading docks. Fortunately, loading docks and public entrances have very different cell phone location signatures, so we can tell which is which.

Cell phone location traces can also tell us that there's something new on the map that wasn't there last month -- a new housing development, a new road, a new business, etc. -- prompting further investigation, e.g. a StreetView vehicle going there to get photographs. The photographs can then be automatically analyzed to determine (in conjunction with cell phone data and volunteer user contributions) what the new thing is.

All of which is one of the reasons Google developed its own phone operating system. I can only assume Apple Maps is doing the same things with iPhone location data.

[cellio]

Thanks for explaining! That all sounds very clever, particularly the part about detecting new things on the map. How well does all that work for less-traveled places, like in smaller towns? (Where it's probably also more expensive to send out a StreetView car to take a look.) Does map accuracy depend significantly on population size?

[hudebnik]

Well-traveled places are already in the database, for the most part, so a lot of the benefit of signals like these is in filling in less-dense parts of the map. But yes, the amount of information we have about a particular place correlates strongly with population density.

Conveniently, many of the people in developing nations skipped the whole land-line thing and went straight to cell phones, albeit perhaps one per village, so we actually have data for places like sub-Saharan Africa.

Sending StreetView cars to sub-Saharan Africa is indeed expensive (although we've got StreetView kayaks, and StreetView backpacks, and there's now annotated StreetView of the International Space Station. Not sure about StreetView camels.) And we can work with satellite imagery when StreetView is too expensive.

[cellio]

StreetView kayaks -- neat! I'd heard about the backpacks (y'all are mapping mountain trails and stuff with them, right?), but I hadn't considered other vehicles.

and there's now annotated StreetView of the International Space Station

*blink* And so there is -- cool!

[jducoeur]

(Curiosity drives me to check) Sadly, no StreetView for Pennsic. But I am amused that Vlad's Pleasure Pavilion not only shows up on Maps as a "Nightclub", it has a five-star rating and four reviews...

[cellio]

No StreetView, but several of the Pennsic roads show up and more encampments than I expected. Drachenwald Royal also has five stars... but AEthelmearc Royal is sadly missing.

The roads have been there for a while. Several years ago the main road to Cooper's Lake was closed (they were widening the formerly-one-lane bridge) and there was a back way in and out. On the way out I used my phone to help with navigation, expecting it to show me in an unmarked patch at first. Nope -- it said I was on Cariadoc's Path. And I was.