Interstate 5 or I5 is an Interstate Highway that runs parallel to the Pacific Ocean and extends all the way from Mexico to Canada. In the Portland-Metro area it’s home to some of the worst traffic snarls and the subject of much debate from commuters, environmentalists and everyone in between.

I myself, am a commuter, meaning that I live in Vancouver, Washington but I work in Portland, Oregon. Unlike some of my commuter brethren, I don’t hold a grudge against people that are anti-commuting; I see their point as equally as I see my own. I firmly believe that a solution won’t present itself until both sides drop the tribal/faction mentality, stop stereotyping and start work collaboratively towards a vibrant and workable metropolitan area.

Until that time, I5 is what it is – a traffic quagmire. Having resigned myself to this realization, I started trying to understand why I5 traffic is this way. Like any good geek, I started this process by collecting data. In a forthcoming article, I’ll describe in detail how I’m collecting the data, but for now I’ll just give a summary. Every 15 minutes, between the hours of 5 AM and 9 PM, an automated script will fetch a speed map of the Portland-Metro area from WSDOT (Washington State Department of Transportation).

Although I’m still early in the process of collecting the data, I found something interesting on July 21st. Although I have no hard data to support this (I wasn’t there and I only have the empirical evidence of the speed maps), I’m fairly certain that an I5 bridge lift occurred in the late morning.

Before we get to the data, I should note, I’m not a traffic engineer, I don’t hold a degree in Civil Engineering, I don’t work for a government agency and the images below are copyrighted by WSDOT. I’m just a web developer with a thirst for knowledge and an intense curiosity for how and why traffic systems work and don’t work.

Let’s look at the data, shall we?

Our story begins at 12:30 PM, traffic on both sides of the Columbia River seems to be flowing normally as evidenced by this traffic shot:

Fifteen minutes later, I5 is at a complete standstill heading southbound from Washington as well as heading northbound from Oregon. Conversely, traffic directly after the I5 bridge – in both directions – is free flowing. This is usually indicative of a bridge lift that allows marine traffic to pass.

Our next shot, 15 minutes later, seems to indicate that the bridge has been lowered. We can deduce this because of movement on I5 south from Vancouver; it is “Wide Open” just before the bridge and “Heavy” just north of that. On I5 north, there is a different story, traffic is actually getting worse as is evidenced by the “Stop and Go” section extending further into north Portland.

At 1:15 PM, on I5 south in Vancouver, we see the “Wide Open” status just before the bridge change to “Moderate” with everything else being “Wide Open”. On I5 north in Portland, we see traffic migrate from “Stop and Go” to “Heavy”; which seems to be a natural progression. You can also see a small band of “Moderate” traffic begin just south of the “Heavy” traffic. This may or may not be unrelated to the bridge lift since it’s separated by a patch of “Wide Open”, free flowing cars.

One hour after we observed the bridge lift, I5 south in Vancouver is free flowing; there are no more slow downs. However, I5 north in Portland still seems to be having issues. Right before the bridge, it’s migrated from “Heavy” to “Stop and Go” and it’s still experiencing “Heavy” traffic just south of the bridge.

At 1:45 PM, heading north on I5 in Portland, the band of “Heavy” transitions back to “Stop and Go” just like it was at 1:00 PM. I5 south in Vancouver remains unchanged.

At 2:00 PM, I5 south from Vancouver is still free flowing, but I5 north continues to experience issues. Right before the bridge, a small band of “Heavy” traffic materializes but the band of “Stop and Go” traffic just after that remains unchanged.

At 2:15 PM, I5 north is beginning to see some small measure of relief as the band of “Stop and Go” traffic transitions, once again, back into “Heavy” traffic. However, all is not well. Just before the bridge, things seems to be getting jammed up again as traffic transitions, once again, from “Heavy” to “Stop and Go”.

At 2:30, all bets are off on I5 north in Portland. The slowdown at 2:15 PM right before the bridge sends a traffic shockwave down I5 south, transitioning the “Heavy” traffic back into “Stop and Go” traffic. If that wasn’t enough, even further south, we now see a new patch of “Heavy” traffic. This new patch is the start of Rush Time traffic (Rush Hour is a misnomer as periods of heavy traffic on I5 north can go on for five to six hours). Because this is now Rush Time traffic, we can stop our sampling and take a look at the results.

So it easier to visualize, I’ve created a time lapse image of the event from 12:30 PM to 2:30 PM.

Ending Observations

It is common knowledge that in the morning commuters will jam I5 south as they head into Portland and beyond. In the afternoon, the inverse is also true, commuters jam I5 north bound as they leave their jobs and head home. This is not a disputed fact. However, it’s interesting that the data shows that I5 north can become quite a mess even when the bulk of commuters are still at their jobs (assuming that the earliest commuters start at around 6:30 AM and leave at 2:30 PM). This is something that’s rarely, if ever, discussed within the commuting debate.

It’s also clear that based on the data, I5 south in Vancouver will recover more quickly from a bridge lift than I5 north in Portland will. Within approximately 60 minutes of a bridge lift I5 south has completely corrected itself. In Portland, I5 north, doesn’t correct itself until Rush Time is over (on July 21st this doesn’t occur until 8:15 PM).

What causes this? I don’t have any data to support my suppositions so I honestly don’t know. If I were to take a wild guess it would be that the arterials on I5 north are poorly designed and this causes and inordinate amount of merging. Merging usually results in some slow down and in turn this slow down translates into a traffic shockwave that can reverberate for miles. I5 north in Portland is not the only culprit, the entrance to SR-14 eastbound in Vancouver is too close and requires drives to slow down to make the turn. During peak times, this slowdown will extend all the way to the Hayden Island entrance on I5 in Portland (this is the last entrance to I5 north from Portland) and essentially turns I5 north from a 3 lane Interstate into a 2 lane Interstate.

For those that are curious as to what happens before and after the bridge lift, I’ve create a time lapse image from the hours of 5 AM to 9 PM.