Something curious happened on the I-405.
In 2015, during the final months of the massive construction project, average speeds during the 4 p.m. – 7 p.m. northbound rush hour were 45.9 mph.
A year later, after all the bulldozers had left, they had slowed to 28.8 mph during the same period. This year the average speed was 27.5 mph.
Transportation planners say, We told you so.
“Even when you add capacity, it gets filled quickly,” said Anne Brown, a professor of public planning at the University of Oregon who has studied traffic patterns in Los Angeles. “The predicted benefits erode over time. You can’t build your way out of congestion. That’s travel planning 101.”
The 405 is already the busiest highway in America, with about 374,000 trips a day. The Sepulveda Pass Improvements Project, as it was called, added a 10-mile carpool lane, realigned 20 on and off ramps and rebuilt overpasses, among other things. The work caused long-term disruption, including two “Carmageddon” episodes, when the route was entirely closed.
Caltrans, the state agency that manages the freeways, says the big drop in commuting speeds after the project finally concluded has a simple explanation: Constant warnings to motorists during the protracted construction convinced them to take alternate routes. When the project was finally completed, the opposite happened: Drivers flocked to the 405, expecting, naively, traffic to flow smoothly. For this article, we only examined data going back to 2015, when construction on the 405 was wrapping up.
“The traffic rebalances itself,” said Mort Fahrtash of the Caltrans traffic management center. “These long-distance commuters spend a good portion their lives on the freeways, so they are intelligent. As soon as a road gets opened they reroute themselves. It’s just a balancing act.”
For our analysis, we looked at morning and evening rush hour traffic data for the period of Feb. 1 – March 25 for the past five years. The stretch of the 405 freeway we measured is 18 miles long between the intersection with the I-10 in the south and the 118 on the north. You can find our broader analysis of 52 freeway commutes here.
Here’s what we found.
Morning rush-hour on the 405 North through the Sepulveda Pass:
- In 2015, average speeds were 55 mph.
- In 2016 speeds jumped to 60.1 mph.
- In 2019, speeds fell to 57.3 mph.
Morning rush hour on the 405 South through the Sepulveda Pass:
- In 2015, during the tail end of construction, traffic moved at 50.1 mph.
- A year later, when everything was completed, it dropped to 39.3 mph.
- In 2019, it popped back up to 45.7 mph. (Caltrans said the roadway system is too complex to attribute a single cause for the change.)
Evening rush hour on the 405 South through the Sepulveda Pass:
- In 2015, average speeds were 51.7 mph.
- In 2016, average speeds edged up to 53.3 mph.
- In 2019, it had slowed to 48.1 mph, a decrease of 7%.
How we did it: We divided freeways into 26 different evening commutes across Los Angeles County. Then, we analyzed the average speeds on those freeway segments between Feb. 1 and March 25 for five years, from 2016-2019. We then calculated the average speeds during the peak of the weekday evening rush hour (4 p.m. – 7 p.m.) and morning rush hour (7-9 a.m.)
Learn more about how Crosstown compiles its traffic data here.
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