1 Potential Issues with Advance Preemption Roelof Engelbrecht Texas Transportation Institute
2 Rail Preemption Designed to transfer right-of-way to the track movement and clear vehicles off the track(s) before the train arrives Preemption sequence in signal controller: Initiated by a signal from the rail equipment Transfer right-of-way to track movements Service and terminate track movements Must be done before train arrives!
3 Current Design Methodology ❶ Use maximum right-of-way transfer time ❷ Assumes a heavy vehicle stopped on track ❸ Calculates time to get that vehicle off track ❹ Track clearance green time set to this value ❺ Request advance preemption time, if needed ❻ Assures vehicle is off track when train arrives
4 Design Case Preempt To Controller Lights Start To Flash Train Arrives At Crossing Warning Time Lights APT Minimum Warning Time (20 sec) Lights Flashing Gates fixed delay Gates Descending Gates Down Signal Queue Maximum RTT Track Green Queue Clearance Time Time
5 Question Is this good enough to guarantee safe operations?
6 Operations Variability impacts preemption operations Variability in traffic signal operations Due to unknown controller state at preemption initiation Affects right-of-way transfer time Variability in rail operations Due to train handling Affects warning times
7 Data Collection Train monitoring opportunities at TTI Instrumented rail corridor in College Station Good relationship with City of College Station Train data collection Logged signal changes Logged preemptions Logged lights & gates Logged train arrivals
Right-of-Way Transfer 8 Time Distribution P(Time is less) 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% P(Already in Track Phase) = 24% P(Needs 5 sec Clearance) = 54% 0 2 4 6 8 10 Right-of-Way Transfer Time (seconds) 5 sec Min. Green + 5 sec Clearance
Preemption Warning 9 Time Distribution 100% 90% 80% Variation: 17 seconds P(Time is less) 70% 60% 50% 40% 30% Guaranteed by Railroad: 20 sec MWT + 15 sec APT Median Value: 48 seconds 20% 10% 0% 30 35 40 45 50 55 60 65 Preemption Warning Time (seconds)
10 Actual Warning Time Distribution 100% P(Time is less) 90% 80% 70% 60% 50% 40% 30% 20% 10% Variation: 5 seconds Median Value: 30 seconds 0% 24 26 28 30 32 34 36 38 Actual Warning Time (seconds)
11 Problem Exists with advance preemption Preemption initiation and warning device initiation are independent processes Based on train arrival estimates made at different times Separated by variable time interval
12 Uncoupling of Events 100% 90% 21 sec P(Time is Less) 80% 70% 60% 50% 40% 30% Preempt to Controller 18 sec Light Start to Flash 20% 10% 14 sec 0% 70 60 50 40 30 20 10 0 Warning Time (seconds)
13 Operations (1) Preempt To Controller Lights Start To Flash Train Arrives At Crossing Warning Time Lights Advance Preemption Time (APT) (variable) Minimum APT (fixed) (variable) Active Warning Time (AWT) Minimum Warning Time (20 sec) Lights Flashing Gates fixed delay Gates Descending Gates Down Signal Maximum RTT Track Green Yellow Red / Dwell Queue Queue Clearance Time Separation Time Time
14 Operations (2) Preempt To Controller Lights Start To Flash Train Arrives At Crossing Warning Time Lights Advance Preemption Time (APT) (variable) Minimum APT (fixed) (variable) Active Warning Time (AWT) Minimum Warning Time (20 sec) Lights Flashing Gates fixed delay Gates Descending Gates Down Signal RTT Track Green Yellow Red / Dwell Queue Queue Clearance Time Separation Time Time
15 Operations (3) Preempt To Controller Lights Start To Flash Train Arrives At Crossing Warning Time Advance Preemption Time (APT) (variable) Minimum APT (fixed) (variable) Active Warning Time (AWT) Minimum Warning Time (20 sec) Lights Gates Uncoupling Preempt trap fixed delay Lights Flashing Gates Descending Gates Down Signal Track Green Yellow Red / Dwell Queue Queue Clearance Time Separation Time Time
16 Preempt Trap Track clearance green may end before warning lights start to flash Vehicles have no indication of impending train arrival and may cross tracks But the track clearance interval has already expired! Worse than having no preemption?
17 Solutions Treat the symptoms and / or Treat the cause
18 Treat the Symptoms Preempt To Controller Lights Start To Flash Train Arrives At Crossing Warning Time Advance Preemption Time (APT) (variable) Minimum APT (fixed) (variable) Active Warning Time (AWT) Minimum Warning Time (20 sec) Lights Gates Preempt trap fixed delay Gates Descending Lights Flashing Gates Down Signal Track Green Yellow Red / Dwell Queue Queue Clearance Time Separation Time Time
19 Treat the Symptoms Preempt To Controller Lights Start To Flash Train Arrives At Crossing Warning Time Advance Preemption Time (APT) (variable) Minimum APT (fixed) (variable) Active Warning Time (AWT) Minimum Warning Time (20 sec) Lights Gates Add fixed delay Gates Descending Lights Flashing Gates Down Signal Track Green Extra Green Yellow Red / Dwell Queue Queue Clearance Time Separation Time Time
20 Treat the Symptoms Provide more than the minimum required track clearance green Balance probability of preempt trap with operational efficiency For example: Actuate end of track green Set track green to APT + 15 seconds Design for measured APT times
Example: Probability of 3+ 21 Vehicles In Preempt Trap 40% P(3+ Vehicles in Preempt Trap) 35% 30% 25% 20% 15% 10% 5% 0% 0 100 200 300 400 500 600 700 800 900 Random Approach Volume (vph) 13 sec Green 16 sec Green 19 sec Green 22 sec Green
22 Treat the Cause Reduce variability in rail operations Reduce variability in advance preemption time Use a not-to-exceed timer to control Max APT Reduce variability in traffic signal operations Reduce variation in right-of-way transfer time Truncate pedestrians (?)( ) to control Max RTT Use a special phase for track clearance to control Min RTT
23 Conclusion Be aware of the potential problems of advance preemption Take the necessary steps to avoid (or at least minimize the probability of) the preempt trap Consider design and operational scenarios
Questions? 24