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| Re: TA is obsessed with CBTC, and ''New'' tech for no reason. (580992) | |||
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Re: TA is obsessed with CBTC, and ''New'' tech for no reason. |
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Posted by Stephen Bauman on Wed Mar 5 07:08:38 2008, in response to Re: TA is obsessed with CBTC, and ''New'' tech for no reason., posted by Jeff H. on Wed Mar 5 00:15:35 2008. Something is wrong with the math, then. The design number which is ACTUALLY used by NYCT in laying out signals is an actual emergency braking distance of 437 feet on level tangent track. Adding a 35% safety gives 590 feet, not 496.I used a spreadsheet, not Excel, so I didn't bother double checking. Let's try it using a calculator. At 40 mph and an average emergency braking rate of 3.2 mph/sec it will take 40/3.2 = 12.5 sec to stop. The stopping distance is: ½at2 or: s = 0.5 x 3.2 x (1.47 fps/mph) x 12.5 x 12.5 = 367.5 ft Add the 35% safety margin: 367.5 x 1.35 = 496.125 feet Emergency braking tests were conducted as part of the NTSB Williamsburg Bridge investigation. The results were included in the report. There were 3 tests: north and south on dry track and south on wet track. The emergency stopping distances from 40 mph were: 358.1; 402.0; and 490.4 feet respectively. N.B. 40 mph was the nominal starting velocity. The actual velocities were: 37.5; 39.5 and 39.2 mph, respectively. Despite the discrepancy between my calculations and the TA's design practices, let's not loose sight of the fundamental assertion. Service levels in excess of 30 tph can be achieved without station timers and with the T/O seeing only green signals. These levels can be obtained even with a signal system designed to thwart suicidal T/O's. The only difference between us is whether that maximum service level is 36 tph or 42 tph. |
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