Diamond interchanges, light timing, and flow priority

[apete2]

In Henrietta NY, most access to Interstates 390 and 590 northbound is served by two roads with diamond interchanges, NY15 and NY15A. Both carry traffic from colleges with significant commuter populations (MCC and RIT) and make up 2 of the only 3 crossings NB over the Erie Canal and I390.

NY15:
http://maps.google.com/?ll=43.111679,-77.623376&spn=0.001852,0.004823&t=h&z=18&vpsrc=6

NY15A:
http://maps.google.com/?ll=43.107398,-77.614675&spn=0.003705,0.009645&t=h&z=17&vpsrc=6

The lights are currently timed to give the green arrow to traffic coming from East River Road (NY15) and I390 Offramp(NY15A) while NY15/A NB has a red light. Only 15 or so cars can make it through the intersection at any cycle, leading to queues 1/2mile long or more NB from 4pm on. The problem for NY15A can start as early as 11am when MCC classes are letting out. Due to the canal and bridge, there is no easy way to widen these to a double left or more. During rush hour, the demand for NB I390 seems to meet the demand for going straight at these intersections and on NY15, it can take 15 minutes or more to clear the intersection from Crittinden Rd.

1) Is this a standard phasing for diamond interchanges?
2) Theoretically, could anything be done here without bridge replacement or a flyover? This setup
http://maps.google.com/?ll=43.206286,-77.674788&spn=0.003699,0.009645&t=h&z=17&vpsrc=6 works great, but I don't know if it would be possible with the given canal.

[Michael]

A diverging diamond interchange might help by increasing left turn capacity and reducing the intersections to two signal phases.  NYSDOT is building one about 2.5 miles to the east on I-590.

[vdeane]

This interchange isn't going to be around much longer:
https://www.nysdot.gov/portal/pls/portal/MEXIS_APP.WEPIDYNPAGEMULTI.show?p_arg_names=p_pin&p_arg_values=439017

[roadfro]

It's tough to say whether the signal timing situation is "standard" for diamond interchanges. The interchange complex isn't a typical diamond interchange, although the 390/15A crossing can function as a diamond interchange with frontage roads.

There are 'standard' signal phasing scenarios that can be deployed at diamond interchanges to help increase efficiency. The common ones are a "3 phase" and "TTI-4 phase". It's tough to explain how they operate in words without diagrams or animations, but suffice it to say that the two intersections have to be timed and treated as one intersection to reduce or eliminate stopping between the two. So there can be a slight increase in delay on the outside of the system (approaching the first signal) but decreased delay on the inside (approaching the second signal).

The 390/15 crossing is not a standard diamond interchange, and appears to function more like a typical 4-leg intersection, except that the left turn to 390 NB is offset beyond the intersection at another signalized intersection. It might be possible to employ some features of diamond interchange phasing in order to have these two intersections function as one...that'd take some interesting analysis to make work...

Diverging diamond would not be an option for either of these locations. While a tempting idea for the 390/15A junction, the current geometry includes through movements to/from NY 15 that a typical diverging diamond cannot accommodate.

[mtantillo]

Here are a few ways to do it. 

1) When you have a lot of cross traffic, and little entering/exiting traffic, then coordinate so the cross street gets greens at the same time.  So when the cross street has green at one ramp, it can cross the overpass and pass through the other signal as well.  When you exit at the diamond interchange, you turn left, and you encounter a red at the other intersection, which you have to wait for it to clear before you can proceed. 

2) When you don't have a lot of cross traffic, and a lot of entering/exiting traffic, then coordinate so someone turning left off the diamond exit ramp does not encounter a red.  Of course, if you are a through vehicle on the cross street, you have a higher liklihood of hitting one or the other signal when it is red. 

3) Sort of a compromise between the two.  Let both signals be green for the east/west main road at the same time to allow through traffic not entering/exiting the freeway to pass through unimpeded.  Then at your northbound ramp have that signal serve the ramp, and at the southbound ramp intersection, have that signal green + green left-turn arrow on the main road allowing people just turning off the ramp to pass right through.  Then reverse...the southbound exit ramp gets a green, and the northbound ramp intersection gets green + left turn arrow on the mainline allowing people turning off the southbound ramp to proceed straight through the other ramp intersection.  Then mainline gets greens in both directions. 

But really, it all depends on how many vehicles are making each movement, which timing pattern is chosen. 

[roadfro]

^ #3 is the basic gist of the diamond interchange phasing I was referring to previously. The 3 and 4 phase patterns do this slightly differently, and one is better than the other depending on conditions.

[vdeane]

It's tough to say whether the signal timing situation is "standard" for diamond interchanges. The interchange complex isn't a typical diamond interchange, although the 390/15A crossing can function as a diamond interchange with frontage roads.

There are 'standard' signal phasing scenarios that can be deployed at diamond interchanges to help increase efficiency. The common ones are a "3 phase" and "TTI-4 phase". It's tough to explain how they operate in words without diagrams or animations, but suffice it to say that the two intersections have to be timed and treated as one intersection to reduce or eliminate stopping between the two. So there can be a slight increase in delay on the outside of the system (approaching the first signal) but decreased delay on the inside (approaching the second signal).

The 390/15 crossing is not a standard diamond interchange, and appears to function more like a typical 4-leg intersection, except that the left turn to 390 NB is offset beyond the intersection at another signalized intersection. It might be possible to employ some features of diamond interchange phasing in order to have these two intersections function as one...that'd take some interesting analysis to make work...

Diverging diamond would not be an option for either of these locations. While a tempting idea for the 390/15A junction, the current geometry includes through movements to/from NY 15 that a typical diverging diamond cannot accommodate.

Might want to clarify... exit 15A on I-390 is for I-590.  Took me a while to figure out that you meant NY 15A (exit 16B).