I'm just an observant amatuer, no professional traffic experience, although, someone linked a manual for LA's traffic management system (it's a commercial system, but the manual is customized for LA), and reading through some of that confirmed the observations of capability I've made over time.
I haven't seen a time until green? (or is time until yellow/red?) sign like that personally, so I'm not sure about usefulness. I'd imagine electricity cost is pretty negligible, assuming LED lighting etc.
I used to drive frequently in areas with countdown timers for pedestrian cycles; if the vehicle cycle was also ending, the vehicle light would turn yellow when the pedestrian signal hit zero. As a driver in the opposing direction, seeing the countdown helped me decide if I wanted to wait for the signal or make a right turn on red (if conditions permitted). As a driver in the direction of the countdown, it let me plan for a yellow, and I think I was able to make better decisions about stopping or going on yellow; I imagine some drivers might take the countdowns as a queue to speedup to make the light, anecdotally I didn't see that, but you'd need real numbers to decide.
Anyway, if the approach to the intersection is a downhill, as a cyclist, it's nice to know earlier if I need to brake or not, or if I could make it through if I pedal a bit. If there's enough bike traffic to support that, it sounds nice. I don't think you could signal to bikes and not cars --- at least where I've riden, bicycles at reasonable speed are vehicles and should travel in vehicular lanes unless a restricted lane is available and appropriate (bike lanes are restricted to bikes, but bikes are not restricted to bike lanes). Given that, I don't know how you'd signal to cyclists and not car drivers.
I haven't seen a time until green? (or is time until yellow/red?) sign like that personally, so I'm not sure about usefulness. I'd imagine electricity cost is pretty negligible, assuming LED lighting etc.
I used to drive frequently in areas with countdown timers for pedestrian cycles; if the vehicle cycle was also ending, the vehicle light would turn yellow when the pedestrian signal hit zero. As a driver in the opposing direction, seeing the countdown helped me decide if I wanted to wait for the signal or make a right turn on red (if conditions permitted). As a driver in the direction of the countdown, it let me plan for a yellow, and I think I was able to make better decisions about stopping or going on yellow; I imagine some drivers might take the countdowns as a queue to speedup to make the light, anecdotally I didn't see that, but you'd need real numbers to decide.
Anyway, if the approach to the intersection is a downhill, as a cyclist, it's nice to know earlier if I need to brake or not, or if I could make it through if I pedal a bit. If there's enough bike traffic to support that, it sounds nice. I don't think you could signal to bikes and not cars --- at least where I've riden, bicycles at reasonable speed are vehicles and should travel in vehicular lanes unless a restricted lane is available and appropriate (bike lanes are restricted to bikes, but bikes are not restricted to bike lanes). Given that, I don't know how you'd signal to cyclists and not car drivers.