someone wrote: ↑Fri Aug 24, 2018 16:17
Maybe I am misunderstanding a whole lot of stuff, but surely to have green all the way though, the other direction has red all the way through. So this is not fill/release but just left/right with short phases, unless the lights are changing so fast it is more like a disco.
I'm envisaging the junction as being large enough that traffic takes a substantial time to get from one set of lights to the other. During a "fill" phase, the lights to exit the junction are red (in both directions), and to enter are green (from the minor road), red (from the major road's right turns). Traffic enters the junction, in both directions, and as the start of the queue starts to reach the end of the junction, the lights change; so the traffic that entered the junction has a free run out of the other side. This also gives a green light to the major road's right turns, which join the back of the queue of traffic leaving the junction.
Fill/release is substantially different from left/right as the settings on the lights are rotationally symmetrical (as opposed to left/right where a 180 degree rotation changes red lights to green lights and vice versa).
And how can a release phase be longer than the fill one when they occur simultaneously for each direction? If eastbound is releasing, westbound is filling. But it would have a longer phase for that than it has for releasing, which obviously will be wasted as there is no extra space for filling. That may have value for times with a dominant flow, but again then it just turns the whole thing into a form of left/right.
No, with fill/release phasing, if eastbound is releasing, westbound is also releasing. If you leave a "release" phase indefinitely, no new traffic can enter the junction from the minor road, and the junction will effectively form a pair of freeflow rights for the major road.
ais523 wrote: ↑Thu Aug 23, 2018 03:30Left/right… in particular, it seems very hostile to traffic turning right off the free-flowing road (which will have to stop at red lights twice and can only advance at the transition from one phase to the next, meaning that a lot of stacking space is needed inside the junction to stop that movement backing up onto the main road). Compare to fill/release, where right-turning traffic off the main road gets a free run during "release" phases, which can be lengthened if necessary to clear any queue.
And I do not undertand this either.
Under the left/right model the right turning traffic is released onto the bridge (so that capacity is not being wasted) when the left side has a green light. Sure, that does means if it hits a red light on the slip road then it will hit the second one on red too.
Under the left/right model, right-turning traffic off the main road ends up on the "closed"/non-flowing half of the junction. That means that each full cycle of the junction's two phases can only allow an amount of right-turning traffic through equal to the number of vehicles that fit inside the junction. In other words, you need to cycle it quickly to avoid blocking the right-turners. This means that a slow-cycling model can be dangerous if there's a lot of right-turning traffic, as queuing on the major road is something that it's normally important to avoid.
But under the fill/release model the bridge will be filled leaving traffic on the slip road having to wait for a gap to merge. It will likely only allow one or two vehicles out at a time in the brief period when releasing and before the filling traffic gets a green light. Although under your description of both lights being green, such a time would not exist.
With fill-release timing, the right-turning traffic off the main road joins the back of the queue for straight-on traffic during the release phase, and continues through the junction behind it. The release phase can continue until the queue clears, if necessary to prevent the main road becoming blocked.
I guess there's a bit of symmetry here: with left/right, straight-on traffic on the minor road in one of the two directions can be given a freeflow by holding the lights, but this blocks right-turning traffic in both directions; a full phase cycle is needed for the
right-turning traffic to get through the junction (and it can do so without stopping only if the cycle time is comparable to the size of the junction, thus likely fairly short as these junctions are designed with a short distance between the sets of lights). Meanwhile, fill-release requires a full phase cycle for straight-on traffic to get through the junction (and it likewise can do so without stopping if the "fill" phase length matches the size of the junction), whereas right-turning traffic can be given a freeflow by holding the lights.
However, the difference is that with left/right, you can't get all the movements working well simultaneously. Changing from a left phase to a right phase (or vice versa) loses the amount of time it takes the junction to clear, taking the capacity below 50%, so you want the junction to be small so that it clears quickly. That means that right turns off the major road are almost impossible, as right-turning traffic needs to fill the junction during one phase and is released during a different phase. You could try to clear that via making the release phase for the right-turning traffic longer, but that's the fill phase for traffic turning right from the other direction. So if you have a lot of right-turning traffic coming from both directions, you need to change phases quickly (and more quickly the smaller the junction is), meaning you lose much of the benefit of your straight-on freeflow.
With the fill/release model, you have a rotational symmetry in the junction: both straight-ons work simultaneously during the fill/release transition, so although the capacity is again 50% (you have to wait for a fill phase before you can enter the junction), increasing the size of the junction doesn't actually reduce the capacity at all (it just changes the optimal cycle time). Right-turning (off the main road) traffic can only move during a release phase, and follows on behind the straight-on traffic, so in times of heavy right-turning you need to make the release phase longer relative to the fill phase; but this doesn't cause any awkward blockages (the only blockage you get as a consequence of that is for traffic entering the junction from the minor road, which is held entirely behind the junction on the minor road and thus is queuing somewhere where it's reasonably safe for it to queue). And of course, that's allowing both right turns simultaneously, so you never have to worry about forming a queue onto the main road in one direction to clear the queue from the main road in the other direction; because a release phase in one direction is a release phase in the other direction, you never have to pick a direction to favour.