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 – 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.
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
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.
http://www.fra.dot.gov/rrs/pages/fp_817.shtml
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?
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.
Assuming an ideal coefficient of friction of 0.8, it takes the average car about 310 feet. A semi truck is about 40% greater than the average car, or and additional 120 feet (Semi trucks have 10 brakes – not 18). The total estimated distance then, would be 430 to 440 feet (no credit for brake lock, driver reaction time).At 70 mph, we have about 120 ft/sec initial velocity. We can take an average deceleration rate of about 20 ft/sec – thus 120/20 = 6 seconds to stop the “average” semi truck.
This leaves the question that we will never know the answer to – when did the driver decide to stop? Or maybe – did the driver decide to stop?
Well, the details we’ve been told – the truck was a gravel hauler – it was coming from the South with two others behind him – they live in the area – the trucking company employs over 100 drivers.
There were skid marks “the length of a football field”, but the impact was hard enough that the cab was embedded in the side of the rail car.
The highway patrol says they will release the driver’s name later today, more than 72 hour after the accident, now that it is old news
I’m developing an alternate theory – given the long viewing distance, perfect weather conditions and the drivers being familiar with the road.
The train was going 78 MPH – the length of the train is around 1000 feet, which means the train would be blocking the road for around 10 seconds. We know the Amtrak engineer anticipated a crash and threw the train into emergency stop.
So my theory is the driver judged that by the time he arrived at the crossing that the train would have cleared the crossing and the gate back up, so he didn’t brake – but when the train went into emergency stop, the decision by the driver backfired and the train ended up blocking the crossing.
The tracks crossing at a 45 degree angle might also have caused a misperception of speed by both drivers. They are not optimistic about getting accurate speed data from the truck’s speed monitor, if it had a device.
More details:
http://www.sfgate.com/cgi-bin/article.cgi?f=/c/a/2011/06/26/MN811K2V6V.DTL&ao=2
He was pulling two gravel trailers, not one. The skid marks were 320 feet long – but you have to factor in how far behind the cab the final set of wheels is.
There was a similar accident recently at this same spot – that driver steered his truck into into the guardrail to avoid hitting the train. That driver blamed the 45 degree angle as being confusing – but he was coming the opposite direction – with the sun in his eyes and the train coming from behind over his left shoulder.
Looked at the street view you posted earlier, and quickly noted that in such a wide open U.S. Highway the placement of the gate/signal is foolishly close to the track… more of what you might encounter in a tight urban environment with low speeds.
I also wonder if the signal lights were of an adequate size for the environment?
The warning signs on the road are on a bridge over a wash, which is a questionable placement of both the sign and pavement markings, and lack of secondary sign or signals a few hundred feet before the track. Particularly on a U.S. highway.
One of the problems out in the desert with distant mountains is, under the ‘right’ conditions (angle, distance, speed, mirage, etc) a train can appear stationary against the background… in some situations it can blend right into the horizon/mountains.
I’ve often encountered the effect while driving through the low deserts in Arizona.
It’s possible the driver never spotted the train (or saw it moving) until it was too late, it’s also possible he was going faster than 70 MPH out there in the open desert. 😉
I think that is very likely. If the 320 feet skid marks is right, and he hit the train with enough force to embed the cab inside the car – and the now disclosed history of speeding tickets – I’m guessing he was going as fast as the truck was capable of going.