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

[metahacker]

GPS's precision costs a lot of battery. There's an API to ask for location, and the more precise you ask to be updated, the more battery it uses. So most apps opt for a less-precise location and stitch it to road maps, assuming you're at a particular location.

Also, there are many spots where GPS doesn't work as well--it is, after all, relying on Signals From Spaaaaaace that can get blocked by buildings, other signals, weather, or nearby military installations...

[cellio]

Oh, I hadn't realized there was a power effect. Does more accuracy = more pings to the satellites?

Also, there are many spots where GPS doesn't work as well

Right. I occasionally use navigation when going somewhere after work, and I turn it on in the parking garage because I don't want to be fiddling with my phone while driving, and I'm used to getting weird or no directions until I'm on the street. There's also a stretch of a few blocks where it often cuts out because of tall buildings. Maybe the hills in Rome were a problem? (Not tall buildings, at least where we were.) Or maybe ancient gladiator ghosts think we moderns are soft and they're disrupting the signals. :-)