CBTC vs Conventional Signalling – Which is Safer
CBTC vs Conventional Signalling – Which is Safer :This post is based on a collaboration with Ali Edraki, VP of Transit & Rail Systems at Gannett Fleming, for a presentation we delivered at the 2016 Rail Safety Seminar in Orlando Florida.
A special thanks to Daniel Sandu of Gannett Fleming for providing valuable insights and technical details about conventional signalling; without which I would not have been able to conduct a proper comparison.
The safety record for conventional signaling is beyond doubt; 140 years of improvements, handed down by thousands of engineers have ensured the safety of our urban transit infrastructure.
However, transit authorities are switching to CBTC in increasing numbers and the primary motivation is due to the operational superiority, not safety, of a CBTC solution over a conventional one.
CBTC shines because it pushes the operational envelope (shorter headways) while maintaining the proper level of safety. Whereas a conventional system is handcuffed operationally due to its inherent limitations (fixed block signalling philosophy).
In this post I will compare the key safety and operational characteristics between a CBTC and conventional system. By the end of this post, it will be clear why transit authorities are selecting CBTC for it operational benefits.
Comparison of Safety Characteristics
The safety capabilities of a CBTC over a conventional solution is not as great some may believe. CBTC does provide additional protections but a conventional system is safe but it relies on a human to follow the rules.
The table below lists the basic safety characteristics expected of any signalling system.
Speed Supervision â€“ Conventional signalling has no speed supervision other than to rely on the driver to maintain the speed within the speed limit for the block. Cabs signalling has become popular over the last few decades, but it has a limited number of speed transitions (usually 3 or 4) permitted for a block. CBTC monitors speed continuously and if the train exceeds the permitted speed, the train borne unit will either emergency brake or service brake the train. Second, there is no limit to the number of speed transitions along the track.
Train Localization â€“ A conventional system utilizes track circuits to detect the location of a train. The resolution is based on the size of the block; either the block is occupied or unoccupied. CBTC localizes the train through a two-way train to wayside communication link that transmits the exact position with a resolution in centimeters. Both methods are safe but CBTC is more efficient from an operational perspective.
Safe Train Separation â€“ Conventional signalling operates on the principal of a one block (or two signals) separation between trains. If there is a failure, the one block guarantees the train will emergency brake to a stop before the next train. A CBTC system keeps a minimum safety distance between trains. If there is an emergency brake condition, the minimum safety distance ensures there is enough room to stop a train before the next train. Both methods are safe but operationally, CBTC is more efficient.
Rollback Protection â€“ A conventional system has partial rollback protection; A) If a train is stopped at a station and rolls back, the train will not prevent the rollback. B) If the train is on an uphill grade commanding effort from the propulsion unit and the train detects a rollback, the propulsion unit will increase power to prevent that rollback. C) Newer trains determine the travel direction based on the active cab. If the train moves in the opposite direction, the train emergency brake. In CBTC, if the train moves in the opposite direction of the route set under the train, the emergency brakes will be applied. If the train moves when there is no route set, emergency brakes will be applied. CBTC provides better rollback protection.
Parted Consist Protection â€“ If a train breaks apart, a conventional system will track the train through the occupancy of the block and the apply emergency brakes. A CBTC system will create a protection envelope around the train. Neither approach is better than the other.
Train Door Interlocks â€“ by definition, the basic interlocks that must be performed are: A) Train is aligned within an acceptable tolerance. B) There is a platform on the side where the train doors will open. C) Train is stopped. D)The train is prevented from moving. A CBTC and conventional system implement these basic interlocks but a conventional system relies on the driver to make the right decision (decision to open doors and which side). Whereas in a CBTC system the human element is removed.
Departure Interlocks â€“ For a conventional and CBTC system, the train will not depart if the train or platform doors are open.
Route Interlocks â€“ A conventional and CBTC system both have approach and routing locking concepts but approach it from different angles. Both methods are safe but CBTC allows for greater throughput through the interlocking than a conventional system.
Protection against Passing a Signal at Danger â€“ A conventional system has trips stops or devices that can emergency brake a train if the train passes a signal at danger. CBTC has no trip stops but if a train passes its permitted stopping point, the train borne unit will emergency brake the train.
Broken Rail Detection â€“ This is one area where a conventional system has an advantage over CBTC. Conventional uses track circuits to detect a broken rail. CBTC does not have this ability unless a secondary broken rail detection system is utilized.
Protection against Human Error â€“ A general principal within the safety community is, the less human interaction, the safer it is. A conventional signalling system depends on the operator to follow the rules of the railroad. If an operator violates a safety critical procedure (speed limit around a curve for example), safety is compromised. Whereas a CBTC system is automated and the human element is removed or reduced significantly.
Comparison of Operational Characteristics
Out of the 11 safety features discussed above, 3 favor CBTC, 1 favors conventional and the rest are neutral. But when comparing the operational characteristics, CBTC has the overwhelming advantage.
Maximize Throughput â€“ Throughput is the primary reason why CBTC is selected by transit authorities. Its ability to reduce headways is unmatched by any conventional signalling technology available on the market today. (read my post about the moving block vs fixed block signalling)
Equipment & Maintenance â€“ A track littered with signals, trip stops, track circuits, associated cables and row upon row of relay racks in the equipment rooms makes for a daunting maintenance program for an operator in any conventional system. The requirements for corrective and preventive maintenance are high and access to the track is difficult; maintenance is conducted during the wee hours of night when the system is closed to passengers traffic. Equipment in a CBTC system is significantly lower and therefore the space required in the equipment rooms and along the track is also low. As a result the maintenance program is minimal (unless itâ€™s a conventional system with a CBTC overlay). More important, the need to access the track is reduced.
Automatic Speed Regulation (Ride Quality) â€“ Uniform ride quality from one train to the next is a staple of a CBTC system. A CBTC train will travel at the same speed and brake at the same rate at the same point on the track every time. The acceleration and braking is gradual and jerk is reduced or eliminated when departing or arriving at a station. In a conventional system, the passengers are left at the mercy of the driver. The driver may accelerate suddenly or brake hard, drive in excess or slower than the posted speed. The ride quality will differ from one driver to the next.
Bi-Directional Operations â€“ A conventional system is designed for maximum efficiency in the normal running direction. Due to cost and complexity, the reverse running direction is not optimized and the headways are large. Options available to the operator in recovery situations is reduced. However, for a CBTC system there is no concept of normal or reverse running; the design is optimized for both directions. Only the civil design of the track limits the efficiency of the system (the placement and orientation of the cross overs and curves).
Reduced wear and tear of train propulsion and braking system â€“ A CBTC systems ability to regulate speed and acceleration reduces the stress placed on the braking and propulsion unit; when compared to a human driver in a conventional system. A CBTC train operates within the design limits of the braking and propulsion unit maximizing its life while reducing maintenance costs.
Energy Optimization â€“ this is the ability of a signalling system to regulate the energy utilized during operations. CBTC optimizes the energy usage by controlling when the trains accelerate (energy consumption spikes when a train accelerates) and allow the train to coast more often when travelling between stations (the propulsion unit is disabled reducing the energy consumed). A conventional system does not have this capability.
Interoperability â€“ This is where CBTC is the weakest. A conventional system is not wedded to any single supplier. A transit authority can procure parts from multiple suppliers and theyâ€™re interchangeable. Whereas, once a CBTC supplier is selected, the transit authority is at the mercy of that supplier. Meaning, if a system has a Siemenâ€™s CBTC system installed, An Alstom or Thales train controller cannot be used in that system. Standards have not been established to allow interoperability between suppliers. This will change over the coming years as CENELEC, IEEE, AREMA define standards and transit authorities force the suppliers to comply.
Automatic recovery from Perturbations â€“ CBTCâ€™s bidirectional capability is the magic behind the recovery from perturbations and its built in safety shines in these scenarios. When an operator in a conventional system operates the system outside of its normal operating conditions, mistakes happen due to human error. In a CBTC application everything is automated, including recovery scenarios. In high congestion scenarios, CBTC excels because it manages the situation as part of its normal functionality (conflict avoidance zones, route optimization, adjusting station dwells and max speed limits, etc.) whereas a human might be overwhelmed with information overload.
Many assume a CBTC system is safer than a conventional system but this is a misunderstanding. CBTC does offer some safety advantages but a conventional system is safe; and 140 years of proven operations demonstrate this. But a conventional system cannot match the operational advantages of a CBTC system and to transit authorities, this is what attracts them.
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