Understanding road intersections

Two whole years ago tomorrow, I wrote in this corner about "Those famous, or notorious, U-turn slots." Of course, I was referring to the Metro Manila ones, forced into the streets of Metro Manila by the equally famous, or notorious, Bayani "BF" Fernando, the erstwhile MMDA chief of the previous administration. Frankly, I didn't expect the same would be copied in Cebu. My first reaction was to ask myself, how did the men and women of the Planning Division of CITOM, trained by the National Center for Transportation Studies in UP, came to this solution. Unless policies changed, the CITOM Board usually consults them.

Now this will get technical, and I mean too technical, but humor me, please. A few days after the implementation of the U-turn slots in Ban-Tal, we got mixed reactions. Some say it improved traffic. Of course, it did. Others say otherwise, that traffic worsened. Yes, it does that, too! So we get to the proverbial question, "Ano ba talaga, kuya?" The answer is, like many other physical phenomena in this life, "it depends." The important thing is, we understand the mechanics, and two, we understand that it's a case-to-case (here, on a per-intersection) basis.

Let's start with capacity: what is the capacity of a one-lane road? The first factor should be the speed of the vehicles in it. Theoretically, a single lane road with vehicles speeding at 30 kph should carry twice the number of vehicles per hour than another where the vehicles creep at 15 kph. In actual conditions, that's not exactly true, since the space between vehicles vary. That's why we have the term "bumper to bumper" in heavy congestion. But when you're cruising at 60 kph at the South Coastal Road, bumper-to-bumper is a sure formula for instant multi-vehicle collisions. Especially when you enter the tunnel portion, but that's another topic. At faster speeds, you maintain a safe distance from the next car. We call that "headway." That's why half the trucks you follow bears a "Keep Distance" sign in its behind.

In a city road network more characterized by intersections, the other peculiar phenomenon is the wave theory of traffic. Imagine a line of 10 cars in an intersection with a stop sign. When the signal turns green, the 10 cars don't move together at once. The one in the front goes first, followed by the second, then the third, and so on. If the line is long, those farther back may not even reach the intersection in time before it turns red again. Then, the "accordion" type of movement occurs in reverse, as cars try to stop as close to the one ahead.

The basic theory of an intersection is that of physics which says, "No two objects can occupy the same space at the same time. And so vehicles take turns crossing the intersection. Applying the above (headway and wave theory), we readily see that congestion almost always happen and are more acute in intersections rather than in the long stretches of roads. Ah, well, they do too, in the latter, but that's simply because of the congestion in the next intersection. The only exception is when there's a traffic accident along the road, or when the road design was really bad in the first place. But intersections usually are the culprits.

There are ways to mitigate the effects on intersections, more geometric than anything else - channelize it, flare it, do a roundabout (a British term we usually call a "rotunda"), like Fuente Osmeña or the one in Carcar, and/or install traffic signals. The last one is the more common, of course, and Cebu had the first "automated" one in the country. We're more American than British and many Filipinos don't even know what a roundabout is. We use "rotunda" which actually means any building which has a circular footprint. Here, it's synonymous to roundabout.

A U-turn, the way it's used in Metro Manila and in Ban-Tal, is an improvised, albeit substandard, rotunda. So maybe the real question to ask is, which is better, a rotunda or a signalized intersection? Ah-hah, now that's something readily "searchable" in the internet, just don't click on those TV shows which simulate real life experiments with no real analyses. As I said last week, transport science is a graduate studies discipline and there's a long cache of professional papers, theses and dissertations about the subject which we can refer to. Try the following websites: Eastern Asia Society for Transportation Studies (EASTS) at http://www.easts.info/, and NCTS at http://ncts.upd.edu.ph/.

When we understand the mechanics of rotundas and signalized intersections, then we can understand more fully how BF and Ban-Tal's U-turn slots behaves. This we will discuss in the next column: "Understanding U-Turn slots." Especially on the matter of "perception vs. reality." Again, there are existing technical papers on the matter readily available. All we have to do is search on the net.