Writing

Software, technology, sysadmin war stories, and more. Feed
Wednesday, May 29, 2013

Reversible lanes and reconfigurable intersections

There's a street in Santa Clara which has been around for a long time and can't expand. It's hemmed in by a bunch of houses and businesses on either side. It apparently moves a lot of traffic in one direction in the morning and then another direction in the evening. The city did something unusual with this street to make this work.

Center lane

For about a mile, there are signs over the road: one for each of the three lanes. Most of the time, the center lane has the usual "two left turn arrows" which means it's a center turn lane. But, twice a day, that lane shuts itself down and then becomes a travel lane. I don't have a picture of the signs in that state, but just imagine a big green arrow on one side and a big red X on the other.

Big deal, right? They do this on bridges and stuff all over the place. Okay, that's true. A reversible lane in and of itself is not too special. What makes this wacky and weird is what happens at one end of this arrangement. There is a place where a relatively big road (De La Cruz) dumps off at Lafayette, and it's one-way, with three lanes across at the intersection.

De La Cruz/Lewis/Lafayette

[Click to see a bigger version of this one.]

This picture shows Lafayette in the distance. If you turn left at that intersection, you enter the beginning of the reversible section. You can also continue straight across or turn right.

Looking at the pavement markings, you might think that one lane can turn left. You'd actually be correct most of the time. When the reversible lane is flowing the right direction, the intersection reconfigures itself to allow two left turn lanes. Did you notice the arrows overhead? Here, take a closer look.

Left and straight

When I took this picture, it wasn't quite time for the afternoon traffic pattern, so it was still set for one left turn lane. The next lane over has to go straight. When Lafayette has the center lane active, a second arrow lights up on that sign, and the second lane can now go straight or left.

You can't really tell from that picture, so here's a picture of another sign at the same intersection. If you look closely, you can see the outlines of the extra left turn arrow.

Left and straight... for now!

(I like the "on green signal only" part. When else would you go? It's not a one-way to one-way situation, so left on red isn't possible here.)

Think about this in practical terms. Most of the time, if you're in the #1 (far left) lane coming down De La Cruz, and you turn left on Lafayette, you end up in the right lane. You don't turn left into the center turn lane.

However, during the afternoon/evening hours when the center lane is actually moving that direction, if you make that same turn, you'd better finish it in the center lane, since someone from the #2 lane beside you might be turning at the same time! They have to go into the right lane, so you'd better head for the center if you don't want to crash.

I don't know how this intersection manages to work without constant side-swiping, but somehow it does. I think people just get really paranoid about each other since it's so tight and they just creep along and figure it out. If there was more room to maneuver, there would probably be more wrecks, in other words.

To make things a little more interesting, there are also other, smaller side streets in this section. At least one of them sports a nice warning sign advising people to avoid that center lane in the afternoon.

Franklin at Lafayette

Unfortunately, this one requires quite a bit of situational awareness. One, do you know what time it is? Two, do you know whether North is right or left? Locals should have no problem with this, but if you're visiting or just new to the area, look out!

Finally, I don't have pictures of this last quirk, so you'll have to use your imagination. One of the intersections has a couple of black boxes which look completely out of place most of the time. However, when the center lane is unavailable, these boxes light up red to say NO LEFT TURN. I guess that's the only way to keep traffic from backing up.