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What COVID-19 teaches us about fixing freeways

Portland Freeway during the blue hour

As everyone knows, traffic is moving much faster today than six weeks ago. Stay-at-home policies have reduced car travel by more than half in most of the nation’s large metro areas.

With lower traffic volumes, cars are zooming along, with most freeway traffic in Portland, Oregon (as just one example) regularly exceeding the posted legal speed limit.

So you might think that our freeways are carrying fewer cars at the peak hour than six weeks ago. But you’d be wrong. Actually, at the peak hour, between 4:30 and 5:30 p.m, major Portland-area freeways are carrying just as many cars now (actually slightly more) than they were pre-COVID-19. And they’re going much, much faster.

Let’s take a close look at Interstate 5 through North Portland, reputedly one of the most congested roadways in the state (and a place where the Oregon Department of Transportation wants to spent upwards of $1 billion to widen 1.5 miles of roadway).

We looked at the area between the Rose Quarter and the Interstate Bridge (milepost 300 to milepost 309). We pulled together real-time traffic speed and volume data recorded by Portland State University’s PORTAL traffic counting program, which measures traffic speeds and volumes. We looked at peak hour travel volumes and speeds northbound between 4:30 and 5:30 p.m. weekdays between the Marquam Bridge (Milepost 300) and the Interstate Bridge (Milepost 308).

Let’s first look at the baseline: What is traffic like on I-5 on a typical weekday? We focused on the afternoon commute because it's generally the worst and most regular traffic tie-up. Consider the PORTAL data for I-5 for the five weeks prior to the implementation of social distancing measures, from Feb. 3 to March 12. On a typical weekday, this stretch of freeway carried about 26,900 cars during this hour at an average speed of 25.6 miles per hour.

Now look at the post-COVID period. Here are the data from March 13 to March 28. To no one’s surprise, speeds are up, to an average of 53.6 miles per hour (on a stretch of freeway posted with a 50-mile-per-hour speed limit). And the peak hour volume, from 4:30 to 5:30 p.m.: It is 27,450 vehicles in this hour, not down, but up, about 2 percent from pre-COVID levels.

Post-COVID-19: Speeds are up on I-5, and so is peak hour traffic volume

The simple but seemingly amazing fact is that the I-5 freeway is carrying more vehicles, faster at the peak hour than it did prior to COVID-19 travel restrictions. How can this possibly be the case?

What highway engineers know, but won’t tell you about congestion

It’s actually a well-known fact that roads carry more traffic, faster, when they’re not allowed to become congested. Freeways are subject to a kind of "tipping point" — they carry more traffic at reasonably high speeds (45-50 miles per hour) right up to a point where traffic flow becomes saturated. When one minor slowdown is rapidly propagated through the system, the road declines in speed and — this is the important part — it loses capacity: t actually carries fewer cars.

The reason that I-5 was so jammed up before the COVID-19 pandemic and is so fast-moving (and is carrying even more vehicles) now is entirely explained by the fact that we’ve kept traffic below that tipping point. The data from PORTAL shows, before and after, how this works.

BEFORE: Tipping point hit at 2 to 3 p.m. and speed falls until 6 to 7 p.m. This is exactly the phenomenon we see on a daily basis on I-5. The chart below shows typical daily traffic volumes from midnight (0 hour) through the entire course of the day. These data are for I-5 northbound from milepost 300 to 307, from Feb. 10 to Feb. 14. The blue lines correspond to speed (measured on the left axis); the blue columns are traffic volumes, per hour, measured on the right axis).

As you can see, about 2 or 3 p.m. every day, traffic volumes on I-5 North reach the saturation level, and travel speeds fall sharply to around 20 miles per hour. They stay that way usually until between 6 and 7 p.m., when traffic volumes fall enough (fewer cars and trucks entering from the south) to allow traffic to accelerate back up the the speed limit.

AFTER: Traffic stays below the tipping point all day; speed averages 55-60 miles per hour all day. The chart below shows the same data as before, for April 13-17.

Because the freeway never becomes saturated and doesn’t push past its tipping point, travel speeds stay high — and the road carries even more traffic at the peak hour (between 4:30 and 5:30 p.m.) than it did prior to the pandemic.

Notice that while peak hour volumes are up slightly, total daily traffic counts are, in fact, down. Consider the 24-hour traffic counts for milepost 307.9. For the weekdays from Feb. 17 through Feb. 28, this stretch of I-5 northbound averaged about 46,700 vehicles per day. For weekdays of the week March 23 to 27, traffic volumes had declined to about 38,100 vehicles per day, a decline of about 18 percent. What this almost one-fifth decline in traffic meant was that at no point during the day did the volume of traffic push the freeway past its usual "tipping point" that triggers congestion and prolonged slowdowns.

How is that possible? The science of freeways and congestion

Here’s the way freeways really work: As long as everyone is moving at 45-50 miles per hour, the freeway works well, as it carries the maximum amount of traffic. But once a freeway becomes even slightly over-loaded, traffic speeds fall abruptly, and — this is critical — the freeway actually loses capacity (it carries fewer cars past a given point at any period of time).

The relationship between traffic volume and traffic speed is "non-linear." When we chart traffic speeds against volumes, we get a curve showing how traffic jams form. Below is an example from the Washington State Department of Transportation’s 2018 Congestion Report. Speed is shown on the vertical axis; the volume of cars on trucks on the horizontal axis. Each dot corresponds to an observation of the number and speed of cars on this section of roadway. The shaded curve shows the statistical relationship fitted to the pattern of dots.

At low levels of volume (top left), traffic moves a long at a free flow speed (usually the speed limit, or slightly higher). As traffic volumes increase (moving along that curve to the right), speeds decline slowly. Once traffic becomes saturated — on this chart, between 1,500 and 2,000 vehicles per lane, per hour — we get to a point of instability, where the speed/volume relationship is erratic and the curve becomes backward-bending. 

The exact tipping point for any stretch of freeway will depend on a number of factors: whether the road is straight or curved; the presence of on- and off-ramps; and other factors. Once a road reaches and slightly exceeds its capacity, traffic suddenly slows, and the road then carries fewer cars. The roadway tends to stay stuck in this lower part of the curve until traffic entering the freeway drops, and cars can accelerate to get to the upper part of the curve.

A lot of the 'improvements' that highway engineers make (expanding the capacity of ramps) actually makes it easier for more cars to get onto the roadway, causing it to jam up sooner.

The reason we have recurring traffic jams is because there’s nothing to stop too many cars from getting on the freeway, and once they do, pushing the freeway past its tipping point, the roadway tends to stay congested. In fact, he "improvements" that highway engineers make (expanding the capacity of ramps) actually make it easier for more cars to get onto the roadway, causing it to jam up sooner. As we documented at City Observatory, the widening of I-5 at Victory Boulevard a decade ago was associated with lower throughput over the I-5 bridges at the peak hour).

That’s why "removing bottlenecks" is a hopeless game of infinite whack-a-mole. When you enable more cars to travel faster at one point, you simply overload the next downstream point even more quickly, and the traffic jams recur. It's one reason that widening freeways never reduces congestion; even when you have 23-lanes of freeway, as in Houston.

So the trick in making a freeway maximize its throughput is to keep traffic levels just below this tipping point. And that’s exactly what the COVID-19 travel restrictions have done on freeways throughout the nation. (It’s draconian and inefficient to be sure, but it shows how managing demand can make congestion disappear).

Policy implications: Managing demand is the key to reducing congestion

The experience of the past few weeks should be a powerful object lesson in how to effectively fight traffic congestion. If we manage demand for the roadways at the peak hour to keep traffic volumes below the tipping point, we can get our existing roadways to carry even more traffic than they do now at speeds right up to (and even above) posted speed limits.

You’d think with this revelation in hand, highway departments — especially cash-strapped ones, such as ODOT, that face a 20-30 percent decline in gas tax revenues — would be looking to figure out how to apply the demand management solution so aptly demonstrated here on a daily basis. And remember, this doesn’t mean a dramatic reduction in total traffic, it just means keeping the volume at any one time from exceeding the freeways carrying capacity.

But what we hear from ODOT is just the sound of more bulldozers. Even in the face of demonstrable evidence about how to solve congestion without spending a dime on concrete or asphalt, they’re reading to march forward with a $1 billion freeway widening project on I-5 at the Rose Quarter — one which, without pricing or other demand management, simply will increase congestion and pollution.

 

The trick in making a freeway maximize its throughput is to keep traffic levels just below this tipping point.

What this really shows is that their protestations to the contrary, highway engineers actually don’t give a damn about traffic congestion. Who builds a multi-billion dollar capital asset and then does nothing at all to manage its use, and in fact, allows it to regularly become monumentally inefficient by being overwhelmed by demand on a daily basis? 

A whole range of measures are available to traffic engineers, if they chose, to make the system work more smoothly and deliver a better experience for all users. As economists have long pointed out, even a modest price for using the roadway at the peak hour would prompt some users to defer trips, take other modes or take other routes — and actually would increase our well-being.

Congestion pricing: What the COVID-19 traffic experience demonstrates is that pricing the roadway would not be an additional cost for road users, but rather it would generate huge benefits in terms of faster travel and better use of the roadway. It's a powerful argument for road pricing.

A system of time-varying tolls for the freeway system would encourage some people to rearrange their trip taking in a way that always kept the freeway from exceeding its tipping point. In this way, pricing provides what the British call "value for money" — if you pay a peak hour charge, you get a road system that works better for you and everyone else.

Managing demand: Pricing may be the best and most powerful tool for making roads efficient, but it's far from the only one. Every freeway in the region is by definition "limited access." Most freeway ramps in Portland have "ramp meters" — traffic signals that restrict the number of cars that can get on the freeway.

The logic of ramp metering is drawn from the same underlying observations about the unstable, non-linear speed volume relationship shown above. Ramp meters are designed to keep too many cars from crowding onto the freeway all at once and producing these jams. But most ramp metering is timid, with fixed intervals (one vehicle every 5-10 seconds, regardless of how backed up or free flowing the freeway is). 

Tougher metering, including allowing traffic to back up on on-ramps in order to keep the freeway moving, would produce higher levels of traffic flow). Likewise, the highway agency could consider simply closing some ramps at the peak hour. One logical candidate is the I-5 Jantzen Beach on-ramp to northbound I-5. This short, sub-standard ramp essentially lets traffic from the Jantzen Beach shopping center clog up the flow of traffic onto the I-5 bridges and is a major source of the initial delays that trigger daily slowdowns. It’s also no coincidence that the improvement in I-5 traffic conditions coincides with the big decline in shopping trips to Jantzen Beach.

Here, in the midst of the COVID-19 pandemic is an illuminating natural experiment that shows how, with a little ingenuity in the form of demand management, we could make our transportation system work much, much better, both relieving the annoyance of daily congestion and providing even more throughput on our expensive infrastructure.

If we’re smart, we’ll learn from this experiment and choose to manage our road system in a way that provides benefits for everyone, rather than simply throwing more billions at wasteful road-widening projects that do nothing to resolve the fundamental cause of traffic congestion.

This article originally appeared on City Observatory.

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