April 09, 2008
More on Katy tollway congestion pricing
I've said before that I don't have a problem with the concept of congestion pricing, but there's something about this that doesn't feel right to me.
Four toll lanes that will open on the rebuilt Katy Freeway in October will become clogged with traffic unless Commissioners Court imposes congestion pricing during peak travel times, county and state officials said Tuesday.
County Judge Ed Emmett said congestion pricing likely will be needed to help the county fulfill an agreement to keep traffic moving at least 45 mph in the toll lanes.
"We don't know how to maintain this (traffic flow) without congestion pricing," said Gary Trietsch, district engineer for the Texas Department of Transportation's Houston district.
The court is expected to set the rate in the coming months. The Harris County Toll Road Authority recommended that passenger vehicles pay $1.25 to travel between Texas 6 and the West Loop during nonpeak hours and that the price double during peak hours and other times when the traffic is moving slower than 45 mph.
Six years ago, the county, the Metropolitan Transit Authority and TxDOT agreed to cooperate on widening the 11-lane Katy Freeway to 18 lanes. As part of that pact, the public bodies committed to operating toll lanes that move at least at 45 mph, providing people an incentive to pay to use them.
To me, the goal should be optimal mobility for the entire system, not just for the toll lanes. Doesn't it make more sense to ensure maximal flow on all lanes? Will they lower the toll if those lanes are zipping along at 65 MPH and the free lanes are all clogged up, or is this a one-way function only?
I understand that HCTRA set this threshhold in order to ensure a return on its $500 million investment. Metro would like for its commuter buses to travel at a decent speed as well. I get that, I'm just saying that it would be better to let the toll lanes slow down to 40 or even 35 if it meant the free lanes got an equivalent bump in speed. But that's not the priority here, so this is what we'll get.
Posted by Charles Kuffner on April 09, 2008 to Planes, Trains, and Automobiles
You're going to see more of this if Emmett and Radack remain in office after this November's election.
You carry more traffic at 45 ish than you do at 35 ish.
The speed vs flow curve is basically a parabola turned on its side. Flow is your x axis and speed is your y axis. Different roadways have different flow profiles, but I suspect HCTRA has a wonderful amount of data for Houston area facilities.
Here's a sample graph http://www.fhwa.dot.gov/policy/ohpi/travel/qc/images/figure5.gif
Basically the object is to set the toll rate such that you can stay out at the far right edge of the relationship.
I suspect Houston area roadways have a much larger number of speeds associated with even higher flow rates out past 2200+ passenger cars per hour per lane (pcphpl) given our habits of driving high speeds and low headways (i.e. tailgating at 55+ mph).
If they've chosen 45 mph its based on some data like that =) (I hope)
Charles, the current HOV lanes EACH carry the same # of people as 3 regular traffic lanes. There is no way, as you say, to ensure maximal flow on all lanes that would make people happy at all. The uplift from having commuters choose a fast-moving vanpool or METRO park & ride bus is just so great... if the HOV/HOT lane wasn't so fast, no one would be on the buses / vans, and the collective system would suffer.
OK. I haven't been reading your posts all that long, but I want to know, "Whatever happened to the concept that, once the toll roads were paid for, that 'we' would be able to travel them for free?!?"
In addition, why are 'we' always getting hit with the reality of new tolls on a road that is still under construction ... and is, in definition, a FREEway!!??!!