When Freeways Cross, Everything Gets Tangled

Why Interchanges Cost Us Time and Money

Jenny Lower

In a city whose traffic snarls consistently earn national and global attention, Los Angeles’s freeway interchanges are the black holes.

Since 2012, average rush-hour speeds across the city’s worst interchanges have dropped by more than half. While Los Angeles traffic in general has worsened since 2012, the poorest performing stretches almost invariably cluster around the places where two freeways cross, and they’re also where declines in speed have been the steepest.

But the city isn’t getting wider freeways. The state’s Department of Transportation, Caltrans, which oversees the state’s freeway system, is instead relying on a modest array of solutions it says can help lift congestion around the city’s worst trouble spots. Humans, it turns out, are terrible at optimizing their driving behavior for consistent traffic flow. All the rapid braking and acceleration that take place at interchanges become a nightmare when you throw several million people together. But Caltrans hopes to mitigate that problem with selective adjustments.

“Nothing is easy about competing resources,” says Allen Chen, a senior electrical engineer with Caltrans in downtown Los Angeles. Widening a freeway to add a carpool lane is expensive and disruptive. Rather, the agency is looking at tweaking the interchanges and connectors to improve accessibility during peak times.

An analysis by the University of Southern California’s Integrated Media Studies Center and the Annenberg School of Journalism reviewed data from more than 20,000 sensors placed on freeways, arterial roads and public buses across the city. The project represents the largest data-driven exploration to date of how L.A. traffic is evolving.

Some of the findings are discouraging. Average rush-hour speeds in the most congested freeway stretches—typically those shortly before and after interchanges—have plummeted by about 25 to 50 percent over the last five years. (Rush hour is defined as 7-9 a.m. and 5-7 p.m.) As merging cars thread their way between oncoming vehicles and exiting drivers swerve across four lanes to catch a vanishing off-ramp, sudden braking can create shockwaves of congestion that lead to lengthy backups.

Take the morning southbound route on the I-405 through the San Fernando Valley, approaching the I-101 interchange around Victory Boulevard. In 2012, average speeds there hit about 33 mph, slowing as cars approached the interchange. Three years later, average speed at this same point had dropped to 18 mph. And in 2016, traffic crawled to 12 mph—a one-third drop in just the last 12 months.

The findings from the data echo broader economic trends: Los Angeles County’s unemployment rate dropped by more than half during the same period, from 11.8 percent to around 5 percent. More jobs mean more people commuting to and from work.

There is no sign of traffic letting up, either. Over the last year, average rush-hour speeds at the worst interchanges have dropped from two or three miles per hour all the way up to 13 mph. On the eastbound I-10 during evening rush hour, average speeds just after the 405 interchange tanked from about 25 mph in 2012 to just over 12 mph this year. The smallest declines may not sound like much, but they amount to double-digit percentage drops when you’re already crawling by at 15 mph. And they become significant when multiplied across dozens of lanes throughout the city.

Chen says Metro has run out of options when it comes to certain spots, like the morning 405 commute through the Valley. “We have done everything operationally possible,” he says, adding, “The capacity is absolutely reaching the maximum.” There are no obvious alternative thoroughfares in that part of town, and the high incidence of accidents on that stretch of the 405 contributes to about half the congestion, he notes. Last November, voters approved Measure M, a sales-tax increase that will generate billions of dollars for transportation projects, including a proposed tunnel under the Sepulveda Pass, one of the biggest trouble spots for the 405. But it will be years before workers break ground.

But in other areas, Caltrans can deploy what it calls “realignment” techniques. These strategies won’t take clogged freeways back up to full capacity, but they can help offset congestion at the worst times. For example, building longer on- and off-ramps gives cars more time to get up to speed or decelerate so they have minimal impact on the vehicles behind them. Installing meters on freeway on-ramps spaces out the flow of incoming cars so they join the freeway flow as efficiently as possible. And a simple solution such as replacing stop signs at the bottom of off-ramps with traffic signals—thereby providing exiting cars with a break in oncoming traffic during rush hour—can prevent backups onto the freeway, Chen says.

Understated, localized fixes like these won’t erase the underlying problem of too many cars on the road. In the long run, improving interchange traffic requires a strategic rethinking of how Angelenos get around and investment in the kind of transit infrastructure Measure M will address.

But they're a start.