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

Re: Predicting the future is hard

[cellio]

Thanks for the thorough explanation!

Is "why are the signs wrong" a FAQ in your industry? (For light rail/subway systems I assumed that they mostly weren't wrong, barring train failures -- limited access and you know exactly where the trains are right now -- but I haven't used such systems enough to have meaningful experiences.)

Re: Predicting the future is hard

[(Anonymous)]

I really don't know much about public perceptions on the traffic side, but where I live, people *love* to tweet photos of the train prediction signs showing something other than what they want. Some the time, this is not incorrect predictions but correct predictions of large gaps in service, which is immediately photographable thanks to the sign. Of course, unlike drivers, they can take such photos safely while waiting.

The nature of disruptions on rail is indeed different. You don't have nearly as much volume-based congestion, since every vehicle out there is, at least in theory, there because a schedule said it should be. (However, at junction points, where trains have to take turns going through, they can experience non-linear congestion effects if they're on high-frequency lines and even one falls behind schedule.) So, absent disruption, the travel times are generally less variable than those on the roads. On the other hand, when a single train breaks down, has accommodate a sick passenger, or has to stop for any other reason, nothing's getting by without crossing over onto the opposite tracks, resulting in significant, difficult-to-predict disruption in both directions.