This post is based on the reader understanding basic and advanced traffic signal controller overlap operations. These are outlined at:
http://ntcip-unleashed.blogspot.com/2012/02/basic-overlap-information.html
and
http://ntcip-unleashed.blogspot.com/2012/02/advanced-overlap-applications.html
Trucks decelerate and accelerate slower than other vehicles. The operation described here provides a method to allow the signal to operate such that when a truck approaches a signal at a point where the signal may normally gap out, or max out, the truck will have a better chance of getting through the signal on a green, reducing the frequency of truck stops.
http://ntcip-unleashed.blogspot.com/2012/02/basic-overlap-information.html
and
http://ntcip-unleashed.blogspot.com/2012/02/advanced-overlap-applications.html
Trucks decelerate and accelerate slower than other vehicles. The operation described here provides a method to allow the signal to operate such that when a truck approaches a signal at a point where the signal may normally gap out, or max out, the truck will have a better chance of getting through the signal on a green, reducing the frequency of truck stops.
In
this example, the truck is approaching the traffic signal on a continuous 10% uphill
grade. The continuous 10% uphill grade
breaks to relatively flat right at the signalized intersection. Any large vehicle stopping would have
increased difficulty in getting moving on a green indication due to the grade
of the street.
This
is not a sales document. There are a
variety of methods that can be used to determine the classification of the
vehicle. This method uses a Wavetronix
HD Count station, located approximately 400 feet from the stopbar down the hill
from the traffic signal. The HD Count
Station uses RS-485 communications from the count station to the traffic signal
cabinet. The processing is done within
the traffic signal cabinet.
More information about the HD Count Station can be found here:
In
the cabinet, there is a Wavetronix backplane, which provides the power bus and
the RS-485 com for the Click! Devices on the back plane. This backplane also provides power and
communications for the Wavetronix Matrix stopbar detection system.
One
key componenet is the Wavetronix Click! 512 vehicle alert module. This module was developed for overspeed
conditions, not necessarily for truck priority.
The Click! 512 module allows the user to program 4 unique channels of
output, where the combination of the HD Count Station and the 512 module will
identify a large vehicle, and make different actions based on the speed of that
large vehicle.
More information about the Click! 512 Vehicle Alert module can be found at:
When
a large vehicle is progressing towards the signal, and passes the HD Count
Station, the Click! 512 module determines the speed of the large vehicle, and
then places a contact closure output to a Wavetronix 114 detector card. The relation of the speed range, and which
input to the controller is shown on the table below. These are starting points, based on the HD
Count Station being 400 feet from the stopbar.
Controller Det
Input
|
Speed Range
|
Lower Speed
|
Ph 12 Extension
Timing
|
17
|
More than 40 mph
|
60 fps
|
7 seconds
|
18
|
30-40 mph
|
45 fps
|
9 seconds
|
19
|
20-29 mph
|
30 fps
|
13 seconds
|
20
|
15-19 mph
|
22 fps
|
18 seconds
|
The extension timing is the controller’s extension timing, such that the controller detection input will call and extend for the time listed, based on one or more truck priority calls.
The
traffic signal controller operates in USER mode (a modification from the
standard 8-phase dual quad operation).
The USER mode is configured such that phase 4 and 12 are sequential,
running to overlap 12, through load switch 4.
Phases 4 and 8, and phases 8 and 12 will terminate simultaneously. This means that when the signal is getting
close to gapping out, or maxing out, and a truck approaches the count station, the
signal will transition from phase 4 to phase 12, and hold the green for the
approach with the truck, while also holding the green for the opposing thru
movement.
The
phase sequence diagram looks as follows:
If
additional trucks arrive while the signal is running in phase 8 and 12, the
signal will continue to hold the green until either phase 12 gaps out, or the phase
12 max timer is met.
If
there is no traffic in the opposing left turn, or on the side street, then the
controller may cycle between 4 and 12, based on the actual traffic on the main
street. Even though the signal is
cycling between phase 4 and 12, the drivers will not see a change in the state
of the indications, since 4 and 12 are run on an overlap through the load
switch for phase 4.
It
is important to keep the min green time for phase 12 relatively short. The detection inputs for phase 12 are set to
call and extend. If the signal is
operating in phase 4 and 8, and ready to gap out, and a truck approaches the
intersection via the count station, the count station system places a call to
the controller on the specific detection channel associated with the approach
speed. The following will occur within
the controller while displays attached to load switch 4 stays green.
- Phase 4 times the yellow interval (typically 3 to 4 seconds, depending on what the engineer sets it to)
- Phase 4 times the red interval (0 to 2 seconds, depending on what the engineer sets it to)
- Phase 12 times the min green interval
- The remainder of the phase 2 extension timing occurs
If
the min green time is excessive, the signal will hold for a longer time than
necessary. For example, if the truck is
going 40 mph uphill, the call and extend time on phase 12 is 7 seconds. This should get the truck to the stopbar at
40 mph. If the signal is ready to end
phases 4 and 8 and move on to 2 and 6, the controller will stay green for load
switches 4 and 8, while the call / extension for phase 12 causes the signal to
hold that green through the timing of phase 4 yellow and red plus the phase 12
min green time. If phase 4 yellow is 3.2
seconds, and phase 4 all-red time is 1.8 seconds and the phase 12 min green
time is 5 seconds (10 seconds total), the signal will hold green 3 seconds
longer than necessary for the truck.
However, if the truck is traveling at 20 mph with the same settings, the
extension timing will hold the green for 8 seconds longer than the sum of phase
4 yellow and all-red plus phase 12 min green.
There
needs to be a balance of the timing that will need to be set by field
calibration.
If
the signal is to be in coordination, Phase 12 will need to be given adequate
time in the split divisions to function within the coord plan.
This is precisely how we serve transit signal priority green extension requests. The bus will call/extend both phases 4 AND 12 so that phase 4 can, if time allows, serve the extension request without maxing out. This allows users following the bus to continue to extend phase 4, rather than get cut off because phase 12 gapped out.
ReplyDeleteI use a phase 12 min green of 2 seconds. It is never served without following phase 4.
This barrier structure allowed a yellow trap with the Econolite ASC/2 controller. I had antibackup enabled for protected/permitted left turn phases 3 and 7. This feature does not function as expected when there are three phases in either ring within the concurrent group. (If phases are 3 and 4, then barrier, it works properly, but with 3, then 4, then 12, then barrier, backup IS allowed.) The solution is some write-protect changes or a jumper that omits phase 3 if phase 4 is green.
You have mentioned that "The extension timing is the controller’s extension timing, such that the controller detection input will call and extend for the time listed, based on one or more truck priority calls."
ReplyDeleteBased on the NTCIP, how will you trigger that controller detection input?