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]

an automated system that only bumps one unit at a time.

That is...a bit of a jump in inductive reasoning. The systems have seemed to me to be very sluggish in updating, which makes me wonder if they're time-delayed. It's possible the sign subsequently changed to 25 minutes shortly after you passed it.

The ones around here, when they're not saying "USE YA BLINKAH!", are often hopelessly optimistic--I wonder if they're programmed to make people feel good about the traffic ahead, rather than provide accurate data. (And if you do notice the discrepancy, you get mad at the sign, not at your fellow drivers.) But this seems far to clever.

[cellio]

I was wondering if the software weren't very bright, implementing "slower" and "faster" one tick at a time until it matched whatever the input signal is. No particular reason other than the suspicion that they haven't put much thought or effort into it. (I've only ever seen them change by one, but I haven't seen a lot of changes either -- normally you'd have to be quick and lucky to notice them, after all.) Maybe it's a time delay. I wonder what the input is.