Why the lights on freeway onramp can't end traffic jams

Jul 06, 2023

It’s hard to enter a highway in Los Angeles County without encountering a stoplight at the end of the entrance ramp — a pause that’s supposed to ease the crush of rush-hour traffic.

But like many Angelenos, West Valley driver Liza Olson wonders what, exactly, those lights are accomplishing.

“Have you ever sat at a freeway metering light that’s red while hardly any cars zip by? Have you ever driven through a freeway metering light that’s green only to join gridlock? What gives?” Olson asked in a recent email.

As it happens, the lights are governed by roadside computers that rely on sensors in the pavement, as well as actual human beings who monitor the system for breakdowns and extraordinary circumstances. And according to the state Department of Transportation, the lights do, in fact, get people to their destinations faster. But you probably don’t notice the improvement because it is, shall we say, not dramatic.

The Times asked Wahib Jreij, senior transportation engineer at Caltrans, to demystify ramp meters. Here is his explanation of how things work.

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The first thing you have to understand is that the metering system is designed with more than just highway congestion in mind. The meters can’t allow so many cars to back up on the ramp that they interfere with traffic on the roadway behind them.

The second thing is what you might call the Law of Conservation of Vehicle Matter. Meters do not add or subtract from the number of cars trying to get from Points A to Points B. When the highway already has more cars and trucks than it can accommodate — hello, Los Angeles! — reducing the number of additional vehicles won’t magically cut commute times.

And finally, remember that machines break. Complaints about malfunctioning meters go up when it rains, Jreij said, because vandals and repair crews sometimes leave delicate electronics vulnerable to the elements. That’s yet another reason not to drive when it rains.

There are more than 1,000 ramp meters just in Los Angeles and Ventura counties, the area overseen by the Caltrans District 7 transportation management center at the intersection of the 134 and 2 freeways in Eagle Rock. (Appropriately, the center is tucked between an onramp and an offramp.)

“All the freeways you can think of, they all have ramp meters,” Jreij said.

You’ve probably noticed a fair number of left-turn signals on city streets that remain steadfastly red until a car or two lines up in the left-turn lane. That’s because the lights are controlled by a computer, which is triggered by sensors under the pavement in the turning lane.

Ramp meters work like that, only with sub-pavement sensors in three different locations. These loops, as traffic engineers call them, feed data to a computer in a roadside cabinet (the “ramp metering station”).

“Occupancy” loops placed under each highway lane measure traffic speeds by gauging how much time it takes a car to travel a short distance. This is how the meters know how congested the highway is at any given moment.

“Demand” and “passage” loops near the metering lights let the system know that a car is waiting to enter the highway, trigger the timer that determines when the lights will change, and count the cars entering the highway. And “queue” loops near the bottom of the ramp tell the system when the ramp is backing up to the point where it could interfere with traffic on the street behind it.

Using data on traffic volumes, road conditions and ramp capacity, an engineer will set the outer limits of the ramp metering station: how little congestion is needed to turn on the stoplight, and the maximum amount of time the meter can make drivers wait to enter the highway. Metering will start when the occupancy loops in the highway reach a threshold for vehicle speed set by the engineer; the more that highway speeds decline, the longer the meter will make cars on the ramp wait for a green light.

A green light typically lasts two seconds (allowing one car to pass per green, which is the most common approach), and a red light must be at least that long, Jreij said. That means the quickest cycle of lights takes four seconds — half go, half stop — which translates to 15 cars per minute per lane. At the other end of the scale, the cycle takes 20 seconds — two seconds green, 18 seconds red — which means only three cars leave the ramp per minute.

Again, how long the red light stays on is determined mainly by the occupancy loops, which feed data to the metering station every 30 seconds. But don’t forget the queue loops! If the ramp becomes too backed up, the queue loops will tell the metering lights to shift to a faster cycle regardless of how badly congested the highway may be.

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Jreij said the ramp metering stations are programmed to handle the sort of congestion you’ll see on a day-in, day-out basis, not worst-case scenarios. That’s where the humans come in. For example, he said, he might have to turn off the meters in the face of a major accident, allowing more cars to jam onto the freeway to lessen the effect on surface streets.

“An incident on the freeway doesn’t mean it’s just on the freeway. It impacts the whole corridor,” Jreij said. “That 5% of the time where you have major incidents impacting the whole corridor ... a human response is needed.”

To help on that front, the Caltrans transportation management centers have video feeds from the highways in their turf, some of which they share over the internet. You can check out the public video feeds on the Caltrans site.

Budgets being what they are, though, Caltrans won’t necessarily have the resources to intervene every time the ramps need a human touch. Similarly, it may take a day or two for Caltrans to respond to a complaint about a malfunctioning meter. Jreij said the department strives to inspect every ramp meter at least once a year, but the frequency of its checks depends on its staffing and funding.

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When Caltrans put ramp meters on a highway corridor in San Diego in the 1980s, Jreij said, the innovation increased speeds on the highway by 40 mph. That’s what most of us would define as a successful system: transforming a clogged highway into one moving so fast, you can’t drive with the windows down.

Now, however, the demand for space on the highways “totally outpaces supply,” Jreij said. Consequently, he said, Caltrans focuses on moving a whole lot of individuals and packages a little bit faster.

“If I turn on ramp metering on Route 101 and my average speed goes from 10 mph to 12 mph, that’s success to me,” he said. And for drivers? “That’s nothing to them.” (Here’s where you think, “I would love to be able to go 12 mph on the 101.”)

Jreij said that Caltrans has found that ramp meters improve traffic speeds in the range of 1 to 5 mph. “If we can get 2 or 3 [mph], we will be very happy. If we get to 4 or 5, that’s crazy.”

That’s not very comforting to folks stuck in a long line on an entrance ramp, waiting for the privilege of heading down the highway more pokily than they could jog. But at Caltrans, Jreij said, “we’re in the business of moving goods and moving people, not moving cars. ... When you look at the numbers of how many goods, how many people you move across a certain point because you improve the speed by 1 or 2 mph, you notice a big difference.”

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