On 21 Apr, 2013, at 2:18, Don Thomas wrote:
> the 20 seconds (actually 22 seconds) is the starting point.
> Unless the laws of physics change, there is no need to change the 22 second
> starting point. The 22 seconds is not based on length of vehicle, it is
> based on the minimum size vehicle.
Transport Canada's website has the rules for grade crossings, including the following, which describes how the basic 20 seconds (not 22) is increased to the "Departure Time" based on factors such as the length and acceleration of the "Design Vehicle". The extra 2 seconds for equipment response time is added to either the 20 seconds or the departure time.
It seems clear that for crossings where long combination vehicles are expected to operate, that the warning time must be designed for these to accelerate from a stop 5 m before the first rail of the crossing to completely clear 2.4 m beyond the last rail of the crossing. The length and acceleration of the vehicle, plus the driver's time to look both ways, must be allowed for.
The warning time must also allow for such vehicles travelling at maximum highway speed to pass through their stopping sight distance and clear the crossing, and this also depends on the length of the design vehicle.
Long Combination Vehicles are described here as typically a tractor and two 48 or 53-foot trailers, (up to 32.4 m long, not including the tractor).
Canadian Railway-Roadway Grade Crossings Standards
4.4 Stopping sight distance is the sum of the distance travelled during perception and reaction time and the braking distance. Braking distance is the distance that it takes to stop the vehicle once the brakes have been applied.
4.7 Departure Time – 'Design Vehicle'
The time required for the grade crossing design vehicle to pass completely through the clearance distance is the design vehicle departure time (TD). It includes the time required for the driver to look in both directions along the rail line from a stopped position and to move the vehicle completely through the clearance distance.
The grade crossing design vehicle departure time depends upon the clearance distance, the length of the design vehicle, and the vehicle's acceleration.
(a) The descent of the gate arm shall take 10 to 15 seconds and its ascent shall take 6 to 12 seconds.
(b) The onset of the descent of the gate arm shall be delayed a minimum of 7 seconds after the lights of the grade crossing warning system begin to flash, in accordance with Table 4-8. Additional time shall be added to the delay of the onset where required in accordance with section 4.9.
(c) A gate arm shall rest in the horizontal position not less than 5 seconds before the arrival of trains at the grade crossing except where the trains enter the grade crossing at 15 mph or less, in which case the gate arm shall rest in the horizontal position by the time trains arrive at the grade crossing.
19.10 Design Approach Warning Time
The Design Approach Warning Time of each approach to a grade crossing warning system shall be based upon the maximum railway operating speed on the approach. The Design Approach Warning Time shall be the greatest of:
(a) 20 seconds. If the grade crossing clearance distance (Figure 4-1) exceeds 35 ft (10.67 m), the 20 seconds is increased by one second for each additional 10 ft (3.05 m), or fraction thereof;
(b) the Departure Time for the grade crossing 'design vehicle' (section 4.7);
(c) the Departure Time for pedestrians, cyclists, and persons using assistive devices (section 4.8);
(d) the time of delay of gate arm descent, plus the time to complete gate arm descent, plus 5 seconds;
(e) the minimum warning time required for traffic signal preemption;
(f) the minimum programmable warning time of the constant warning time device; or
(g) the time for the design vehicle travelling at the maximum road operating speed to travel from the stopping sight distance (refer to subsection 4.4) and pass completely through the clearance distance.
The time of operation of the flashing lights before a train movement operating at the maximum railway operating speed enters the crossing shall be the Design Approach Warning Time, plus the additional equipment response time of 2 seconds, or the equipment response time recommended by the manufacturer.
19.12 Consistency of Warning Times
(a) Operating control circuits shall provide reasonably consistent approach warning times for trains routinely operating over the grade crossing.
(b) Where the maximum railway operating speed has been reduced, other than for a temporary slow order, the approach warning times for trains routinely operating over the grade crossing, including trains operating at the maximum permitted speed, may be up to 13 seconds longer than the Design Approach Warning Time, but shall not exceed the limits in section 20.4(c).
(c) Operating control circuits shall be designed so that the operation of the flashing lights in advance of the arrival of a train travelling at the maximum railway operating speed does not exceed:
(i) 35 seconds, for grade crossing warning systems without gates; and
(ii) 55 seconds, for grade crossing warning systems with gates.
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