Vehicle-based turn restrictions

[Zmapper]

Suppose an intersection entrance has two lanes leading up to it. Currently, on approach the left lane is signed left-turn only, while the right lane is for straight or right-turning traffic. After turning left, there is a bus lane to the right of the general traffic lane. The left turn movement gets significantly congested at times, delaying bus traffic to the point where an extra bus might need to be added to the route (read: $) in the near future to keep service on time.

As a quasi queue jump for bus traffic, what I want to do is allow buses to turn left from the exiting right approach lane. In other words, the left lane would continue to be for left-turning traffic, while the right lane would be for straight and right-turning traffic, but would also allow buses only to turn left.

How should this intersection layout be signed and marked?

[Kacie Jane]

You can see something similar relatively frequently with freeway on-ramps (particularly in areas that use metering). The leftmost or rightmost lane will be a totem lane for general traffic, and the second lane will either be another turn lane or an option lane for HOVs (often so they can bypass the ramp meter).

Too hard/lazy for me to find a Street View link or draw a mock up, but I'd imagine you could adapt the signage from one of those situations pretty easily.

[jakeroot]

The right lane would have an R3-6R sign overhead, with a supplementary "EXCEPT BUS" plaque. A separate bus signal would be posted on the mast or mast arm, either using louvered signals, or those transit-specific signals that are sometimes mounted on top of other signals:



This type of phasing would really only work at the beginning of a phase. If a bus tried turning left from the through lane during the middle of the phase, you'd have cars getting stuck or two left turn lanes yielding. A bit awkward if not properly designed.

[jeffandnicole]

Being that this appears to be an extremely specific issue, which intersection are you referring to?  Feel free to post a Google Maps link to it.

If the road being turned left onto is at least 2 lane wide, one less confusing option is to do split phasing.  This will allow the specific leg of the intersection you're referencing to have the left lane turning left, and the right lane allowing for left, straight and right movements for all traffic.

Another option: Retime the signal as is.

Suppose an intersection entrance has two lanes leading up to it. Currently, on approach the left lane is signed left-turn only, while the right lane is for straight or right-turning traffic. After turning left, there is a bus lane to the right of the general traffic lane. The left turn movement gets significantly congested at times, delaying bus traffic to the point where an extra bus might need to be added to the route (read: $) in the near future to keep service on time.

I'm a little confused by this.  How does an extra bus keep all the buses on time?  If you add an extra bus, they'll add a timetable for that bus, and if they don't compensate for the traffic light that bus will simply be late as well.

[kalvado]

I'm a little confused by this.  How does an extra bus keep all the buses on time?  If you add an extra bus, they'll add a timetable for that bus, and if they don't compensate for the traffic light that bus will simply be late as well.

Probably something like 4 buses can run 12 minute intervals at 45 minute round-trip; but once round-trip gets congested to 55 minutes - you now need 5 boxes to keep same 12 min interval.

[jeffandnicole]

I'm a little confused by this.  How does an extra bus keep all the buses on time?  If you add an extra bus, they'll add a timetable for that bus, and if they don't compensate for the traffic light that bus will simply be late as well.

Probably something like 4 buses can run 12 minute intervals at 45 minute round-trip; but once round-trip gets congested to 55 minutes - you now need 5 boxes to keep same 12 min interval.

Congestion won't work like that though.

Let's say at 7am everything is fine.  At 7:30am congestion increases, but it doesn't suddenly back traffic up 10 minutes.  There'll be a 1 cycle delay, then eventually a 2 cycle delay, then a 3 cycle delay, etc.  So each of the next round of buses take a minute or two longer to get thru the intersection, delaying the next stops.  Eventually you get to that 10 minute delay, but it doesn't happen all at once.  If you add an additional bus into the mix, that bus will still meet the 12 minute goal at the beginning, but will be delayed at the traffic light, just like the others.  As rush hour runs down, then the buses are delayed less as it takes fewer cycles to get thru the light, and they eventually get back to their true 12 minute interval. 

So no matter how many buses are introduced, if they all maintain the same circuit, they will all be similarly delayed.