Friday, May 19, 2017

Holiday travel charts a mix of facts, figures and common sense

by Barbara LaBoe

We know our holiday travel charts are popular – we get requests for them well before most holidays and drivers were disappointed last year when we were unable to produce Fourth of July forecasts.

Our Memorial Day Weekend charts are now out, but how do we make each year’s forecast? Good question.

Start with data, mix in analysis and common sense
The quick answer is a healthy mix of facts and figures with a dash of common sense. Staff in our Travel Data and Analysis office start out with historical traffic data from our roadway traffic sensors, then add in some analysis to improve the forecast.
We know holidays like Memorial Day have heavy traffic, but analysis and common
sense goes into predicting when the heaviest travel times will be.

An example? Last year there was a sudden mid-day dip in congestion on I-90 on the Saturday of Memorial Day weekend. That didn’t look right to our traffic engineers when they pulled up the numbers this spring, so they did some sleuthing. Turns out there was a large crash which temporarily stopped all traffic. Fewer vehicles moved past our sensors – but only because they couldn’t move, not because there were fewer drivers on the road. Using data from other years, we adjusted the charts to predict the steady congestion we expect to see this year. Without that adjustment, the straight numbers could lead people to think that several hours midday on Saturday will have relatively low congestion, when we expect exactly the opposite to take place.
We know holidays like Memorial Day have heavy traffic, but analysis and common
sense goes into predicting when the heaviest travel times will be.

Once the data is collected and analyzed, staff from our graphics and web teams format the charts so they’re ready to share with the public. We try to release the charts a week before major holidays to give people plenty of time to make plans.

How should I use the charts?
The charts are forecasts of the entire corridor – not an exact spot or milepost. They’re also tools to help in planning, not promises or guarantees. A crash or sudden, bad weather, for example, can change conditions and forecasts rapidly, so be sure you’re always prepared with supplies and check traffic conditions before heading out.

People returning to the west side on Memorial Day should travel
 before 9 a.m. or after 9 p.m. to avoid heaviest congestion.
Our traffic engineers suggest looking at the overall trend the charts show and then plan accordingly. The number of vehicles isn’t as important as the overall arch showing heavy congestion times. Is late afternoon and evening looking congested? Consider leaving earlier in the day or postponing travel until late evening or the next morning. If you have to travel during a peak time, recognize there will be lots of traffic and give yourself plenty of extra travel time so you’re not rushing or distracted by watching the clock.

Historically, our forecasts are pretty spot on. But ideally, our analysts hope that by sharing the predictions they’ll actually be proven “wrong.” If enough people adjust plans and travel during non-peak times, they say, it helps everyone travel more smoothly.

Are charts from previous years interchangeable?
We’re often asked why we can’t use last year’s charts for the current year. This is where the human factor and experience play a role. If the Fourth of July is on a Saturday, for example, we might see most people travel on late Thursday/early Friday and Sunday. If the holiday is on a Sunday, though, Friday night/Saturday morning and Monday afternoon become the most popular travel days. Those are things our engineers factor into their predictions.

A fair amount of traffic heads to Canada on the Saturday of
Memorial Day Weekend but traveling early or late will help.
Why don’t you have Seattle travel charts?
We get this question a lot. The answer is that Seattle is large enough that it doesn’t act like a typical traffic corridor because it has too many different ways to get around. Going north/south, for example, you can use I-5, I-405, SR 99 or a variety of city streets. That makes it hard to predict with good accuracy.

Our charts focus on the corridors where we see the most congestion without nearby alternative routes: I-5 from Olympia to Tacoma; I-90 over Snoqualmie Pass; US 2 between Stevens Pass and Leavenworth and I-5 near the Canadian border. In these cases, those routes are essentially the only viable options.

Why can’t you make the charts for every weekend, or my morning commute?
We’re happy the charts are useful enough for people to want them on a regular basis, but it takes a significant amount of time to do the analysis for each holiday – especially researching anomalies – and we don’t have the resources to do that on a regular basis.

Heading back from a trip to Eastern Washington on Memorial Day?
The roughest US 2 traffic will be from 10 a.m. to about 1 p.m.
If you want to do some comparison yourself you can get an idea of area traffic volumes using our Map Archives page, which lets you search by area, date and time of day. It won’t have analysis or as much historical data, but it can give you an idea of typical traffic at a given time.

What about Fourth of July this year?
Normally, we don’t create charts for holidays that fall midweek, including this year’s Fourth, which falls on a Tuesday. There are many more variables about when or if people choose to travel on those holidays. Last year, a glitch in our system prevented us from producing Fourth of July charts, which disappointed many.

So, would Fourth of July charts be helpful this year – with the caveat that the dates people may travel may not be as clear? Or should we stick to charts only on three- or four-day weekends? Leave your comments on this blog or email to: and we’ll use them as we make our decision.