Light rail transit is lauded as a congestion reliever by proponents, but this column in the Dallas Observer newspaper by Jim Schutz points out a practical problem: Do we have the technology to pull it off?
Rail transit works best--both in terms of speed and cutting downon travel time--when trains work on tracks with dedicated right of way. That's when the trains cruise by cars stuck in gridlock on the highways at peak hours. But, one of the supposed virtues of light rail is that it can also operate at street level. This is where transportation planning gets complicated.
At the street level without grade separation, light rail trains are subject to the same delays as cars--pedestrian walkways, traffic lights, heavy-traffic intersections, etc. To speed up light rail, and further put cars at a disadvantage, planners are increasingly giving trains priority at intersections by letting the trains (or train operators) trigger a green light at the intersection for the trains. So, cars stop, trains go.
This traffic circulation problem is compounded by the fact that mass transit rail networks are (rationally and appropriately) designed on a hub-and-spoke system. The most heavily traveled corridors that are most transit competitive are the spokes leading to the downtown hub.
In Dallas, where the rail system is operated by the Dallas Area Rapid Transit system, or DART, this approach is creating gridlock downtown. As Schutz observes:
"On September 14, when DART opened service on a segment of its new Green Line from Carrollton to Pleasant Grove, all the trains in the entire region slowed to a near standstill, choked by a rail traffic jam in downtown. DART said it was just first-day jitters. Now, they say, everything is back to normal.
"I say we won't see normal again for about seven years.
"I wrote about this last July 23 ("Tracks of My Tears"), and also in April 2008, both times citing DART's own studies predicting havoc if all its new suburban rail lines wind up going through downtown before DART builds a second downtown rail alignment.
In fact, people knew about this when DART first began laying out its rail lines. It was pretty simple. If all of the regional rail lines have to connect through downtown Dallas by passing east and west on the one existing rail corridor on Pacific Avenue, then at some point there will be too many trains trying to get through the bottleneck."
Transportation planners knew about this problem (it's not that hard to forecast), but promised the new lines would be up and running in time to prevent gridlock. They weren't (and aren't). A current DART rail map can be found here.
Schutz has an excellent, if lengthy (the entire article is worth a read), explanation of the problem:
"But the difference is downtown. What I am really looking at is one giant traffic implosion getting ready to happen downtown in December 2011. That's when the final branch of the system, the Orange Line from DFW, comes on line. Here's the deal.
"The trains go up and down Pacific Avenue. Cars go back and forth across Pacific Avenue. If cars can't cross Pacific, vehicular traffic can't cross downtown, especially since our city council in its stupefying un-wisdom has agreed to a pernicious scheme called "signal prioritization."
"DART trains drive on the streets downtown. They have to stop for red lights just like cars. Or they did. Now with signal prioritization the driver of a DART train can push a button and turn the light ahead of him green."
This is all supposed to work because traffic signals delay cars, in theory, about 30 seconds and then give them another full two minutes to restore traffic back to normal. DART is planning on 2.5 second headways for their trains. But, life doesn't work in the real world the way it is planned:
"DART intends for the trains to be sync'd so that two trains—one running east and another running west through downtown—will pass each other at precisely timed moments causing fewer traffic interruptions.
"Morgan Lyons, the spokesperson for DART, explains that to me: "While the traffic signals cycle every 75 seconds (or 1.25 minutes)," he says, "it's often the case that trains in each direction will move through the intersection so you wouldn't always have that many interruptions."
"If everything runs like a Swiss watch.
"But what I've been finding out here the last couple days is that everything is running like musical chairs. In fact, back at the office when I put all of my times in an Excel spreadsheet, this is what I find:
"Trains that are supposed to be running at 10-minute intervals are running at five minutes, seven minutes, nine minutes apart. Trains that are supposed to be running at five minute intervals are running at six, four and three minutes. And almost none of them is hitting the precisely scheduled moment when it is supposed to arrive at a given station.
"If I place myself right by the point where the trains come out of the Central Expressway tunnel and rise to ground level at the east end of downtown, I can see why the timing is so jagged: two-thirds of the trains I am watching slow down and even halt for a few minutes before entering downtown.
"Swiss watch, hell. They're winging it!"
So, if you have light rail coming to your downtown, prepare for gridlock.