Caltrans dreamin’ about interchange

Caltrans announced Tuesday its preferred method for improving the 405-101 freeway interchange, which the agency says is one of the busiest in the world. But don’t get too excited. The agency also says that it doesn’t have the money to design or build the project, which is expected to cost about $135 million.

Still, this is one of those classic bottlenecks, and understanding why it doesn’t work helps explain a lot of other bottlenecks on Southern California freeways.

The big problem is that entrance and exit ramps have been placed too close to one another. The particular issue on the 405 is that traffic trying to merge onto the freeway from the Burbank Boulevard onramp immediately must merge with traffic trying to leave the 405 for the exit to the 101.

That means a lot of weaving and braking, the reason that the area scores low on Caltrans’ measurement of traffic speeds.

Between mid-2004 and mid-2007, that particular portion of the freeway also had a significantly higher accident rate than the statewide average (1.45 accidents per 1 million miles driven compared to 1.09 per 1 million miles driven).

The other big mess is the ramp from the 405 south to the 101 west/north. It’s a very tight turn and only a single lane.

Caltrans says the ramp was built to handle up to 1,500 cars per hour, but during the morning rush on weekdays it averages 1,792 vehicles. The afternoon rush is better, but only slightly, with an average of 1,374 vehicles per hour using the ramp.

Two design flaws are obvious with the current setup: Traffic that is trying to get onto the 405 from Burbank Boulevard has to merge with traffic trying to leave the 405.

Here’s how Caltrans is proposing to fix the problems:

The existing one-lane ramp from the 405 south to the 101 west/north would be replaced with a two-lane ramp that allows speeds of 50 mph. In addition, the ramp would enter the 101 west of the 101’s exit to Haskell Avenue -- so incoming 101 traffic doesn’t have to mix with 101 traffic trying to get off the freeway.

The Burbank Boulevard onramp would be routed under the new 405-to-101 exit ramp, so that traffic won’t enter the 405 south until after the 101 exit. That also eliminates access to both directions of the 101 for those entering the 405 from Burbank Boulevard.

The new two-lane bridge from the 405 to the 101 would cross the Sepulveda Dam’s spillway. Caltrans is saying the bridge would match the dam’s architectural style.

These types of bottlenecks are all over the region, where entrances and exits to freeways were built too close together. My favorite is in Pasadena, where westbound 210 traffic that is trying to exit to the leg of the 210 that continues to La Canada Flintridge and beyond must mix with traffic trying to get on the 210 from four different entrances.

I asked Caltrans spokeswoman Judy Gish if there was a timeline for the project.

“There is no timeline because there are no funds at this point,” she said.

--

Steve Hymon writes The Times’ Bottleneck blog about Southern California traffic and transportation in real time. Check it out at www.latimes.com/bottleneck.