Tuesday, April 12, 2005

Improving managed lanes in Houston

There hasn't been a lot of press about this, but the consensus developing in the transportation world (at least in Houston and Texas) is that the future is toll roads. The gas tax is just not keeping up with the growth and demand. Not just normal toll roads, either, but ones that use "congestion pricing" to vary pricing during the day and keep the road flowing at full speed (by raising prices at rush hour to reduce demand). They go by the generic term "managed lanes". These lanes will also ultimately replace HOV lanes. Busses and carpools will still use them for free, but other vehicles will be able to use them if they pay the toll, so we'll get a lot better utilization out of the lane space than existing HOV lanes.

The simple reason for their newfound popularity is technology. The EZ tag makes it finally possible to implement what has been around in theory for a very long time. There's no room to do toll booths with these roads and lanes - just the electronic readers like the Westpark toll road.

In some cases, barrier separated HOV lanes may be removed and replaced with a managed lane each direction. In other cases, like I-10 and current 288 plans, new managed lanes will be created down the middle.

One of the big problems is safety, esp. if the barrier between managed and free lanes is soft instead of hard. A Dallas Morning News article notes that accidents increase substantially when the barrier is "too soft", the problem being that people jump back and forth between congested lanes and fast lanes. But additional impediments like flexible divider poles (picture with the article) can substantially improve the safety, while also allowing flexibility to move traffic around accident or emergency situations (not possible with hard barriers).

Another problem you'll notice if you go to some of these public meetings on future plans: a lot of right-of-way gets lost to shoulder lanes. The managed lanes, free lanes, and sometimes even the feeder lanes all get shoulders on both sides in both directions. It's super-safe, but adds up to a whole lot of wasted space, which is fine in the suburban countryside but a real problem in core areas of the city with limited right-of-way.

I'm not a traffic engineer, but another option that might be worth considering: could the buffer zone between managed and free lanes be expanded from 2-4 feet to a full-size lane width (or a little more) to not only reduce accidents (more separation between fast and slow traffic), but also act as the emergency shoulder for both the free lanes and the managed lanes, thereby minimizing right-of-way losses to excessive shoulder lanes. Then put those super bumpy reflector rows in the shoulder lane to prevent driving at any substantial speed in them. My crude attempt at a diagram: (visualize a road running from left to right)

  • Hard barrier
  • 2-3 feet of separation from hard barrier (nobody likes to drive right next to the wall)
  • Managed lane(s) --->
  • Extra-wide shoulder/emergency lane with soft pole barriers on each side and heavy-bump traffic-slowing reflector rows - shared by both managed and free lanes
  • Free lanes --->
  • Normal right-side shoulder lane

This solution can reduce 4 shoulder lanes to about 2.5, which, depending on the available right-of-way, might create 2 new traffic-carrying lanes each direction, which would be a very big gain.

1 Comments:

At 10:22 AM, April 14, 2005, Blogger Ring Zero said...

For a long time the study of automobile traffic was modeled as a kind of fluid flow, with roads obviously playing the role of "pipes." The model seemed natural enough.

Unfortunately this model didn't work most of the time. It turns out that cars don't act like water molecules, because of something called a "driver."

Perhaps unintuitively, reducing the speed limit can actually speed up traffic in congested areas. Among the reasons are (a) Lower speeds reduce accidents. One accident in a congested area spells the end to reasonable flow for a significant chunk of the day. (b) At high speeds, the gap between vehicles needs to be larger. Congested areas can't tolerate big gaps. (c) Possibly the biggest reason, believe it or not, is that higher speeds cause more brake tapping. Simulations have shown that brake tapping is devastating to traffic flow. The driver behind the brake tapper sees a brake light, and instinctively brakes himself, usually a bit harder than the tapper. This sets off a chain reaction that can bring flow to a halt. If you've ever come across a seemingly random slowdown where you could find no accident or any other apparent cause, it was probably due to brake tapping.

So, it turns out, enforcing a reduced speed would probably help the traffic situation. What have been shown not to work in all but a few cases are HOV lanes. Apparently carpooling is less tolerable to people than hours in traffic.

I think the EZ Tag toll lanes would probably work. I don't know if there have been any studies on this idea, but I say go for it.

 

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