Highway-Rail Preemption Preemption can be defined as the transfer of normal operation of traffic signals to a special control mode through establishing wired connection between the traffic signal and the railroad control cabinets, also know as 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 MUTCD: preemption warning time 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
8 - Wire Preemption Equipment In the traffic signal control cabinet 2 - Wire
The new interface
In the Railroad cabinet 1027 feet / 35 seconds =20 MPH (train speed)
Detector placement distance (D) The track circuit must be designed to provide a certain amount of warning time at the highway-rail crossing before the train enters the crossing D=WT x Maximum Train Speed WT: Warning Time
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)
Minimum Warning Time (this is a railroad design element) 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.
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
MPT = MT + CT + BT + ERT + APT 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
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.
QCT: is the total amount of time required for a design vehicle to start up, move through & clear the Queue Start up Distance (L) How to compute QCT: If MTCD = 26, CSD = 38 & DVL=75 Find Queue Start up time (time before DV move)= 2 + L = 2 + 26+38 20 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 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 + ST MPT = 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 time MWT = 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 devices APT = MPT - 20
APT = 31.6 S MPT = 51.6 S MWT = 20 S Simultaneous =51.6 S
Interconnect The 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 Circuit Prevents 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 Railroad Panel Energy + 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.
Preemption Interface Railroad Panels Advance Preemption Supervised APR Crossing Active Energy + XR Crossing Active Circuit
Interconnect Circuits Traffic Signal Health Circuit Informs the railroad controller about the condition of the traffic signal system APT = 31.6 S MWT = 20 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 Tignal Preemption 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