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)
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
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
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
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
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