2012 Minnesota DOT – ACEC Annual Consultant Conference

2012 Minnesota DOT – ACEC Annual Consultant Conference

Alternatives to Advanced Preemption at Railroad Crossings


What is Railroad Preemption at a Grade Crossing?


Railroad Preemption:A special control mode in a traffic signal controller designed to start up and clear any vehicular traffic on the roadway approach crossing the railroad tracksThen allows only traffic movements that do not conflict with the railroad for the duration of the train movement


8 3

7 4

25

16

1. Terminate any allowable combination of phases and associated pedestrian movements.


8 3

7 4

25

16

2. Display Green for Track Clearance Movements and hold in Green until railroad gates are down.


8 3

7 4

25

16

3. Allow only those phases or movements that do not conflict with crossing until preemption ends.


What is Advanced Preemption?


Advanced Preemption is an operating mode which allows the traffic signal to display

green to clear the track before the railroad flashing lights

begin to operate


8 3

7 4

25

16

1. Track clearance green interval begins


8 3

7 4

25

16

2. Railroad warning devices begin to operate


8 3

7 4

25

16

3. End track clearance green


That seems simple enough, what’s the issue here?


When preemption is implemented, adequate time must be provided in order to permit a Design Vehicle to

clear the crossing prior to the arrival of the train75’ in MN


So how much time does it

take for preemption to

operate?

In this example, 52 seconds is

required.

101’


Minimum Green = 7 secondsPedestrian Change = 29 seconds

Yellow Change = 4 secondsRed Clearance = 2 seconds

32 seconds of Advanced Preemption Time required at this location


The required amount of time

must be provided by the

railroad train detection circuitry


MUTCD requires that the

railroad must provide a

minimum 20 seconds of

warning prior to the arrival of a through train


Any time required

beyond the minimum

warning time prior to the

arrival of a train becomes an addition to a basic warning system project


In a nutshell..

Time=

$$$


Why?1.Substantial additional circuitry due to electrical design limitations of the train detection equipment2.Railroad train control signal system must co-exist with crossing systems3.Remote equipment locations which may be 1000’s of feet from the crossing4.Extensive underground cabling to remote locations


What are the alternatives to advanced preemption or

lengthy advanced preemption times?


Turn off the traffic

signal

Effective?

Practical?


Grade Separate

the Crossing

Even more $$$


Close the Crossing

Never a popular

choice with local

politicians


Practical Engineering Solutions


Pedestrian TreatmentsCrosswalk Elimination


From our example, the Pedestrian Change Time is 29

seconds

Engineering changes to the intersection can reduce the

time required from the railroad


In many cases, there is no sidewalk present

between the track and the

parallel roadway


Eliminate the crosswalk

parallel and closest to the

track

Can save 2 – 4 seconds of pedestrian

change time


Pedestrian TreatmentsRefuge Islands


Installing pedestrian refuge islands to shorten the distance and

required pedestrian clearance time


This permits the required pedestrian

clearance time to be reduced by

approximately 50% depending on

crosswalk lengths


New TechnologyQueue Cutter Signal


A Queue Cutter Signal is the most cost effective tool

available to reduce advance preemption time


A Queue Cutter Signal is a separate traffic control signal installed at a grade crossing


A queue cutter signal is not connected to or operated as a part of a

downstream signalized intersection


Generally, a queue cutter signal is installed where the Clear Storage

Distance exceeds 400 - 450’


Queue Loops

Queue loops must use fail-safe design and vital loop processor system

Each loop wire has 2 independent loop circuits – a detect loop and a check loop

Queue cutter signal flashes red if there is a loop system failure

Varies200 – 250’

Typical Queue Cutter Signal Layout


Queue Cutter Signal

inLos

Angeles


It is interconnected with the railroad warning system with a small amount

of advance preemption time

This allows time for the queue cutter to change to red before the railroad

flashing lights activate


When is a Queue Cutter not a Queue Cutter?

When it’s a Pre-Signal


Installed like a Queue Cutter

Looks like a Queue Cutter

Too close to the downstream intersection to use queue

detection


A Pre-Signal still requires full Advanced Preemption Time

from the railroad

Why?


Confusing to road users – is it a pre-signal or near-side

signal faces?

Do I wait here or can I make a right-turn-on-red?


Pre-Signal ornear side

signal faces?


CLOSING THOUGHTS

Remember – Preemption operation is not as easy as it appears on the surfaceRecognize – You need professional help to understand and properly design the systemUnderstand - Connecting 2 independent systems produces a third, more complex systemGrasp - The railroad is your friend – REALLY!Plan - It will take longer and cost more than you think.


Questions?

Rick CampbellCTC, Inc.

[email protected]

mailto:[email protected]

2012 Minnesota DOT – ACEC Annual Consultant Conference

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