Crossing gates existed long before computers and integrated circuits. They work using mechanical relays.<\/p>\n
http:\/\/matt.zont.org\/signals\/crossings\/xngworks\/xngworks.html<\/p>\n
That’s a good primer. The oldest type work based on the same principle as block signals. You create an electrically isolated section of track – as soon as the wheels of a train enter that track, it creates a circuit and that activates the gate. It is based on distance from the crossing, not time – making them dangerous for crossings with wide variations in train speed.<\/p>\n
A better (but more complicated) circuit has a Predictor circuit – the detector is further away from the crossing – it measures the rate of change in the signal and uses that to estimate the speed of the train and then predicts the moment in time to activate the signal. An inherent problem with a predictor is it assumes the speed of the oncoming train is relatively constant – if a train (like an Amtrak train) is accelerating after it enters the monitored area, it can get confused and drop the gate too late or drop the gate and refuse to reset it<\/p>\n
Motion detectors are also used in some situations where a train often sits near a crossing but isn’t planning to move over the road – when the train starts to move , the gates need to drop ASAP.<\/p>\n
http:\/\/www.fra.dot.gov\/rrs\/pages\/fp_817.shtml<\/p>\n
That’s the Federal Railroad Administration’s guidelines – it says that 20 seconds is the minimum and 45 seconds is the upper bound where people will start going around flashing gates. Interestingly, it makes no mention of the speed of the vehicle traffic. How long does it take a truck to stop from 70 MPH?<\/p>\n
The document does mention that predictor circuits can be fooled by an accelerating train – and that extra time should be added if the crossing has truck traffic.<\/p>\n","protected":false},"excerpt":{"rendered":"
Crossing gates existed long before computers and integrated circuits. They work using mechanical relays. http:\/\/matt.zont.org\/signals\/crossings\/xngworks\/xngworks.html That’s a good primer. The oldest type work based on the same principle as block signals. You create an electrically isolated section of track – … Continue reading