Rail crossings are a real annoyance for those sitting at them, especially when one is unfortunate to get stuck behind a mile long train going only a few tens of miles per hour. Such prolonged stops can easily introduce produce major traffic snarls, extending the misery, although a passenger train will not produce a delay longer than a typical 90 second red light. Additionally, there safety issues whenever a crossing between two directions of traffic occurs, resulting in the occasional strikes of vehicles or pedestrians by trains, leading not only to deaths of the trespassers, but also delays to passengers aboard the train as well as passenger fatalities in extreme situations. Conceptually, at least, separating rail lines from roads where they currently cross is a good idea.
However, the problem is that these are extremely expensive and generally paid for by rail funds.
Consider, for instance, OCTA's current grade separation projects. Nearly 600 million dollars buys only seven grade crossings with minimal benefit to rail operations. While it does prevent the occasional delay from vehicular and pedestrian strikes, the benefit as seen by rail travelers is minimal to nonexistent. To make matters worse, that sum could easily pay for the electrification of the rail line from Los Angeles Union Station to Laguna Niguel/Mission Viejo Metrolink station with all the accompanying benefits of electrification, including significantly faster acceleration and average travel times, as well as agency benefits from the resulting higher ridership and fare figures and lower operating costs.
Thanks to the positive train control mandate, however, there is the potential to eliminate safety issues arising from most vehicular strikes. With readily available technology, one can use a variety of means, from radar to inductive loops, to determine whether a particular crossing is blocked or free of vehicular traffic. In addition to raising gates to permit the egress of offending vehicles, as Amtrak's four-quadrant gates in Connecticut do, a temporary "mini-block" may be overlaid upon the normal block with the gate crossing transmitting its normal information to the system. With this mini-block system, the engineer is easily notified as to whether any problems such as an obstructing vehicle await at the crossing and may take appropriate action, or even have the train automatically brake in response to a sudden change in situation. With the use of additional safety measures such as medians and quad-gates, this reduces the risk of vehicular strikes to last-second ramming through the gates, for which additional safety measures are available. This additionally prevents accidents arising from human error in crossing repair. To my knowledge, the positive train control system used in Michigan, ITCS, operates in this fashion.
Delays from suicidal or terminally oblivious pedestrians will not, of course, be affected by such a system.
Doing this does require that a train be able to stop in time, of course. FRA regulations require that lights are flashing for a minimum of 20 seconds prior to the arrival of a train, which comes out to a distance of 2,317 feet for a train traveling at 79 miles per hour. Courtesy of this document*, page 21, we have an equation for determining the stopping distance in Imperial units, provided we know the braking rate, which can be estimated for the FLIRT at approximately 2.8 mphps. For our 79 mile per hour train, that comes out to 1,634.5 feet, allowing for a a shade under 6 seconds for complete clearance of the tracks prior the train necessitating a halt. Of course, stopping distance may take longer depending upon equipment and this is only the minimum warning time, with many signals providing a much greater warning period. The point is not to establish a hard and fast rule about how it ought to be done, but merely to demonstrate that it can be done and provide for rail nearly all the safety benefits of grade separation at a fraction of the cost.
*Worth reading if purely for the amusement factor of the Department of Defense spending money to definitively say "No, the TGV is not suitable for hauling M1 tanks."