Where is I-15 busiest? What time of day has most traffic? Answers to your questions about Utah’s busiest roadway.

If you are able to plan ahead, you can avoid chokepoints and rush hours.

(Trent Nelson | The Salt Lake Tribune) Interstate 15, looking south from 3900 South in South Salt Lake, in 2019. The state collects plenty of data about traffic patterns on Utah's busiest freeway.

When you’re stuck in traffic, you have plenty of time to think.

Maybe I’m alone in this, but when I find myself in those backups, I don’t experience road rage. Instead, I slip into more of a meditative state of curiosity: Who are all these people? What are they doing with their lives? Where are they going? Are they happy?

And, admittedly, a selfish thought crosses my mind: Could I have avoided this traffic?

I finally decided to find out. There’s so much traffic data out there compiled by the Utah Department of Transportation that it can be overwhelming. But I wanted to analyze the place where I — and probably you — most frequently get caught in a traffic jam: on Interstate 15 in Salt Lake County.

On Utah’s most trafficked road, what’s going on? Sure, I have my ideas of the most trafficked spots and times, but are they accurate? Are there days that are more likely to result in heavy traffic? Let’s get a brief overview of the vehicular landscape.

Where is I-15 busiest?

As you’d expect, different parts of I-15 have more traffic than others. The impact of various on-ramps and off-ramps, freeway interchanges, and population centers means traffic is more likely in certain spots.

But, of course, it also depends on which direction you’re traveling. Take a look at the graph below, which represents average daily northbound-traveling traffic during 2022. At the bottom of the graph is each mile marker of I-15 within Salt Lake County at which UDOT measures traffic, and then we’ve annotated some important interchanges along the way.

Not only that, but note that you also can select, via drop-down, a specific drive time if you’re so inclined: 6 to 9 a.m., 4 to 7 p.m., or all hours.

When traveling northbound, the most traffic comes in the section of I-15 immediately south of 9000 South. Then, there’s a big drop-off as the traffic heading to Interstate 215 peels off on a separate collector route. Incoming I-215 traffic rejoins I-15 later to create more high-traffic points.

How about southbound?

Going the other direction, it’s just before 9000 South that is most trafficked, with just after that exit also being a frequent problem point — especially between 4 and 7 p.m. Before I-215 and after Bangerter Highway are highly trafficked as well.

What time of day is busiest?

For this question, we need to look at UDOT’s “continuous counting stations” along I-15 in Salt Lake County that record traffic in each lane, each hour, every day. UDOT has hundreds of these overall, on interstates, state thruways, and more, but we’re focusing on stations at 2900 South, 8500 South and 12900 South on I-15. For this data, we’re looking at the publicly released information in 2021 and 2022.

It’s not a surprise, but 4 to 5 p.m. is the most heavily trafficked time at five of our six counting stations — each tallying northbound and southbound movement. At only one station, 2900 South southbound, is the most heavily trafficked time an hour earlier, 3 to 4 p.m.

The shape of that traffic reveals more interesting trends. For example, the northbound graphs have far flatter shapes than the southbound ones. When heading north, traffic is much more similar in the morning and evening than when traveling south.

When looking at the hour-by-hour data, 5 to 6 p.m. southbound traffic is similar to the 4 to 5 p.m. southbound volume at 12900 South, but the 2900 South traffic sees a bigger decline later in the evening.

What day of the week has the most drivers?

I’ve always wondered about this: Which day has the most drivers?

The answer? Friday.

I was surprised, though, to see Saturday to be nearly at weekday heights on I-15. There are just 2,000 more drivers on average on I-15 in either direction on Mondays compared to Saturdays. I wouldn’t have bet on that.

How has traffic changed in the past five years?

OK, I can’t say this was a question I’ve thought about while in my car in traffic, but when looking at the data, I was stunned by how different the traffic patterns were at different parts of the valley. The pandemic clearly impacted various parts of our valley and its traffic in different ways — both while the pandemic was in full throes and in the work-from-home environment since.

In the northern part of the county, the story is fairly straightforward: Traffic has stayed fairly stagnant from 2018-2022, with a logical dip in 2020 due to the pandemic.

But in the southern part, traffic has increased in a very significant way during the past five years.

UDOT has the beginnings of a reconstruction plan in northern Salt Lake County and Davis County for I-15, and that makes sense. But the traffic trajectory — which may well be unusual due to pandemic factors, and is worth watching — indicates that traffic is increasing far more in the southern region.

To be sure, there’s so much more data to explore here: UDOT even has access to “probe” data, which more accurately indicates driver speed through each traffic area. UDOT contractors have access to this data, but not media.

At the very least, after this exploration, I consider myself a slightly more informed I-15 driver. I know which times to avoid in which particular areas on which particular days.

And if those drives are unavoidable? Well, I can get to another meditative state. Hopefully, not one so focused on traffic.

Andy Larsen is a data columnist for The Salt Lake Tribune. You can reach him at alarsen@sltrib.com.

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