Highway-Rail PreemptionPreemption can be defined as the transfer of normaloperation of traffic signals to a special control modethrough establishing wired connection between thetraffic signal and the railroad control cabinets, also knowas interconnect.

Why preempt?

Remember: the train always has the right of way; the preemption control operation is intended to clear vehicles on the approach that is crossing the tracks before the train arrives at the crossing.

Failure results in severe consequences

Where to preempt & how?ADOT TGP 628

Use TXDOT procedure to calculate preemption warning time

MUTCD:

Within 200 feet of crossing, where the intersection is signalized.

Over 200 feet; queuing analysis should be considered.

Currently there are 41 signalized intersections with RR Preemption. Some has two wire connection and

others 8-wire.Nogales, along B19: 6 intersections will be converted

to advance preemption (construction in progress).

There are 35 more intersections to go

Grand Ave & Bell Road: Grade Separation (Design

Build in progress)

I-10 at Ina Rd in Tucson: Grade Separation

(Construction in progress)

Preemption zones, detection sensors and interconnect

Preemption Equipment

In the traffic signal control cabinet

2 - Wire

8 - Wire

The new interface

In the Railroad cabinet

1027 feet / 35 seconds=20 MPH (train speed)

The track circuit must be designed to provide a certain amount of warning time at the highway-rail crossing before the train enters the crossingD=WT x Maximum Train SpeedWT: Warning Time

Detector placement distance (D)

warning time is based on Speed and Distance

MTCD: measured from STOP line, Warning Device or 12’ perpendicular to the track centerline to 6 feet beyond tracks (select greatest value)

MT (Minimum Time) = 20 S.CT (Clearance Time) = MT should be increase by one

second added for every 10 feet of MTCD clearance distance greater than 35’

ERT (Equipment Response Time), were known as adjusted time (AT)

BT (Buffer Time) to account for any uncertainty such as train speed variation.

Minimum Warning Time(this is a railroad design element)

Traffic signals may require more time to terminate phases and clear any queued vehicles off the

tracks

Max. Preemption Time (MPT)(this is a traffic signal design element)

It is determined, requested and paid for, by the owner of the traffic signal system

Maximum Preemption Time

is the amount of time needed following

initiation of the preemption sequence for

the highway traffic signals to complete the

timing of right-of-way transfer time, queue

clearance time, and separation time.

MPT = RWTT + QCT + ST

𝑀𝑃𝑇 = 𝑀𝑇 + 𝐶𝑇 + 𝐵𝑇 + 𝐸𝑅𝑇 + 𝐴𝑃𝑇

The train could trigger preemption at any moment

RWTT: is

the

Maximum

amount of

time needed

for the

worst case

condition to

clear

conflicting

phases prior

to display of

track

clearance

green

interval.

How to compute QCT:If MTCD = 26’, CSD = 38’ & DVL=75’Find Queue Start up time (time before DV

move)= 2 +𝐿

20= 2 +

26+38

20= 5.2 𝑆𝑒𝑐𝑜𝑛𝑑𝑠.

DV Clearance: time for DV to accelerate through DVCD (from figure 2 of Texas Guide) is 13.8 Seconds (DVCD=DVL+MTCD)

𝑄𝐶𝑇 = 5.2 + 13.8 = 19 𝑆𝑒𝑐𝑜𝑛𝑑𝑠

QCT: is the total amount of time required for a design vehicle to start up, move through & clear the Queue Start up Distance (L)

Separation Time, ST: is a buffer between the departure of the last vehicle from the railroad crossing and arrival of the train.

𝑆𝑇 = 4 𝑆𝑒𝑐𝑜𝑛𝑑𝑠

𝑀𝑃𝑇 = 𝑅𝑊𝑇𝑇 + 𝑄𝐶𝑇 + 𝑆𝑇𝑀𝑃𝑇 = 28.6 + 19 + 4 = 51.6 𝑆𝑒𝑐𝑜𝑛𝑑𝑠.

Simultaneous Preemption:

notification of an approaching train is sent to the traffic signal and railroad active warning devices at the same timeMWT = MPT

Advance Preemption:

Notification of an approaching train is sent to the traffic signal for a period of time prior to activating the railroad active warning devicesAPT = MPT - 20

MPT = 51.6 S

APT = 31.6 SMWT = 20 S

Simultaneous =51.6 S

InterconnectThe Agency is required to identify the

desirable preemption manner and the

circuits

Advance Preemption?Or

Simultaneous Preemption?

Interconnect Circuits

Supervised Circuit

Gate Down Circuit

Active Crossing Circuit (XR)

Traffic Signal Health Circuit

Interconnect Circuits

Supervised Circuit

• Supervised Single Break

• Supervised Double Break

Interconnect Circuits

Gate Down CircuitPrevents the traffic signal controller from terminating

the track clearance green interval before the railroad

active warning devices become active and automatic

gates are lowered

Gate Down

Energy +

Railroad Panel

Traffic Signal Cabinet

Interconnect Circuits

Crossing Active Circuit:

To verify minimum APT provided by

the railroad in a varying condition,

such as when a train stops short of

crossing. Once train remain stopped

for about 20 seconds, the crossing

warning devices will de-activate;

hence the preemption request will be

cancelled and the automatic gates

will rise.

Energy +

Railroad PanelsPreemption Interface

Advance Preemption

Supervised

Crossing Active

APR

XR

Crossing Active Circuit

MWT = 20 S

Interconnect Circuits

Traffic Signal Health Circuit

Informs the railroad

controller about the

condition of the traffic

signal system

APT = 31.6 S

Presignal

References

Traffic Signal Operations Near Highway-Rail Grade Crossings. NCHRP Synthesis 271

Traffic Control For Railroad & Light Rail Grade Crossings, Part 8 of MUTCD

Report No. FHWA/TX-01/1439-9 (Guide For Traffic Signal Preemption Near Railroad Grade Crossing)

TXDOT, Guide for Determining Time Requirements for Traffic TignalPreemption at Highway-Rail Grade Crossing

CTC, x-RPS Railroad Preemption System.

CTC, Preemption Interconnect Circuits.

FHWA, Railroad-Highway Grade Crossing Handbook.

Preemption of Traffic Signals Near Railroad Crossings, Ite