Subway stations with long escalators usually have three. There are three escalators in Porter Square in Cambridge, and in all (or most) of the long-escalator Metro stations in DC (those don’t have stairs, even). This is mostly so that when one is shut for maintenance, there is redundancy for the others to run. But when all are operational, generally two run up and one runs down.

There’s a good reason for this: escalators are a natural bottleneck, and people arrive at a station pretty much randomly in a steady stream. But people depart stations in clumps: trains arrive and people get off all together. Even still, there can be queuing at bottlenecks where people have to slow down or navigate something which causes them to change their speed. Like getting on or off an escalator (or stairs).

So, there’s an eclipse on Monday. (I promise this is related!)

• First thing first: the eclipse is totality or nothing. 99.3% = 0%. You have to go into totality, and preferably far enough in that you get more than two minutes of totality.
• Second thing second: avoid clouds, and don’t plan ahead beyond “eclipse day.” In 2017, my father and I were in Louisville with family. The eclipse path there passed from Missouri to South Carolina, and we were prepared to drive any distance in any direction to see it. Turns out that we were able to go towards our intended destination (Chicago) and it was worth it.
• Third: You do not need to go to where a bunch of other people go. Or somewhere specific. No matter where you go, if it’s clear, the sky is the sky. (Okay, climbing Mount Mansfield, which isn’t really allowed this time of year anyway because the trails are muddy, might let you watch the eclipse shadow approach over Lake Champlain at 1100 mph, but also is more likely to be in the fog.)
• Expect spotty cell coverage at best. Rural areas may already be uncovered, but even the towers there might be completely overwhelmed. Pack a map, and snacks.

I’m very excited about seeing the eclipse. It’s a brilliant celestial event which occurs once every few years-if-not-decades with reasonable travel distance, it’s not commercialized, it’s a great communal event. And … I’m almost as excited about seeing what happens to traffic afterwards because this is a once-in-a-lifetime event and we really have no idea how it is going to play out.

Traffic happens because of the intersection of two elements: volume and bottlenecks (in another sense, demand and supply). A lane of traffic has a theoretical throughput of 2400 vehicles per hour, although this is rarely achieved. Once it becomes saturated with traffic, both speed and volume decreases; this process can take place solely based on volume around 1600 vehicles per hour (or a bit higher). Volume drops to 1000 or even less, with low speeds: this is called “Level of Service F.” (Far too often we plan roads around avoiding this by building oversupply rather than trying to manage demand.) Bottlenecks can cause additional queuing, and this isn’t solely lane drops: merges can be just as bad, as can what I call “sorting” where vehicles approaching an exit have to change lanes. Given high enough volume, this can devolve into a traffic jam, even if there is no theoretical loss of roadway supply. Once a road is congested, it requires a drop in demand to become less congested. That may not happen post-eclipse.

In most cases, we have a good idea of how traffic will behave. In urban areas, we see this occur on a daily basis. In more rural areas, there are certain locations which have frequent traffic when city-dwellers (and suburb-dwellers) return from weekend travel. In New England, these are somewhat illustrative:

• I-95 southbound in New Hampshire traffic is the product of two merges and two sorts. First is the 95-16 merge, a 5-to-4 merge. Then is a sort-and-merge for the Hampton Tolls, which were inexplicably built with only two through lanes of toll booths even though 75% of weekend traffic pays electronically. As people move around and squeeze into these lanes, it causes severe congestion. From there you’d expect that the 4 lanes of traffic feeding into 6 (95 south and 495 south) wouldn’t be congested, but with the toll sorting, traffic then has to re-sort to the two destinations, causing additional congestion. (Northbound traffic, while usually not as bad, also has multiple merges and sorts.)
• I-93 between Manchester and Concord. While the toll sort here is also problematic, the main issue is the 4-to-3 merge in both directions, with additional traffic streams joining. So again, a merge impacted by a sort.
• The Cape Cod bridges. 3-to-2 merges, short merge distances, narrow lanes with poor sight lines, etc.
• The Turnpike at Sturbridge. Eastbound it’s a pretty simple 4-to-3 merge. Outbound it’s theoretically 3-to-4 but the sorting into the exit to 84 often backs up miles. (The 290/395 exit often creates its own traffic jams, sometimes these merge: fun!)
• I won’t get into Connecticut but the highways there seem to have been planned by looking at a bowl of spaghetti, with left entrances and exits and merges and sorts which will congest in a slight breeze.

Aside from these, and Connecticut, Providence and Boston, there’s just not a lot of traffic in New England. On some fall foliage days, a few single-lane roadways in New Hampshire can have backups of epic proportions, mostly when a town (or even traffic light) lies between a road and a highway: The Kanc, Route 100, Route 16. These are traffic jams that occur a few days per year; a few miles of highway bypass could reduce them, but there’s other reason to do so, especially when some people are going to “do the Kanc” and don’t mind if it takes 4 hours to look at the leaves.

Once these people high the highway, there’s enough capacity for them until they get to a bottleneck closer to home. So we don’t really know how traffic is going to behave with more people on the roads than ever before. We have a sleepy, low-volume subway station and a once-in-a-lifetime event … and we are all taking the escalator.

This is different than Phish: the Vermont traffic jam then was caused by muddy fields which couldn’t be used for parking, and a backup onto the interstate (a mile of traffic only has about 200 vehicles in it; so this was basically a 30-mile-long stationary queue of cars trying to get to a parking lot that didn’t exist, until everyone abandoned their cars and walked to the show). That was caused by everyone trying to get to a single point (that wound up having limited capacity). The eclipse covers thousands of square miles. Many people will get there with time to spare (given how booked-out hotels are, days in advance for some). But once it’s over, it’s over. It’s on a Monday. Everyone is going home. We’ll converge on the same roads. The usual bottlenecks in New Hampshire may see some traffic. But there are new ones further north which have never seen this much traffic.

Vermont doesn’t have many continual traffic counters. There’s one just south of Barre/Montpelier. Peak traffic there is about 1000 vehicles per hour on weekdays, and up to 1500 per hour during peak season (foliage, ski). This is significantly lower than the capacity of a two-lane roadway (3200 to 4000 per hour). The one near Waterbury is similar, there’s a bit more traffic right near Burlington at commute times. These roadways never operate at even 50% capacity. So we have very little idea how they will operate when at or over capacity, particularly when these flows hit downstream bottlenecks.

The AADT (average annual daily traffic) on I-89 ranges in the 20 to 30 thousand range, other area highways are far lower (in New Hampshire, I-93 has an AADT in Franconia Notch of 11,000, and under 7,000 a few miles north in Vermont). During peak summer and foliage weekends, I-91 in Vermont sees as many as 8000 vehicles per day. The busiest stretches of roadway in Boston see that many vehicles per hour in each direction. There are some expectations of 200,000 people going to Vermont for the eclipse. This is a week’s worth of traffic on I-91 and I-89.

So around 3:15 on Monday, or a few minutes after, we’re going to see all sorts of new bottlenecks and people flow onto highways for the trip south. Where will it be worst? Here are my predictions/guesses:

• I-89 south of Burlington. Burlington traffic will fill the road, which then runs along the south side of the path of totality, so additional traffic in Waterbury and Montpelier will attempt to merge on; and this is a road with relatively high baseline traffic.
• White River Junction. The ramps here are not designed for peak traffic traffic, although the likely peak demand (89S-91S and 91S-89S) luckily do not overlap.
• RIP anyone attempting to get from Vermont to New York on Route 7, single lanes and traffic lights do not have particularly high throughput. 87 in New York may not be much better and I wonder what the ferry lines will look like (although it would be a great place to watch the eclipse).
• Franconia Notch. I-93 looks like an Interstate, but was built to non-Interstate standards through Franconia Notch after various legal wrangling. With 10,000 vehicles per day, the single lane merge never really backs up. With most of the Boston area converging on it at the same time, well, I’ll be avoiding it.

We have some data to go off of from 2017, when there were similar traffic jams around a very similar event. I tweeted out some traffic photos, shown here are people headed out of major population centers (Atlanta and Charlotte) towards the eclipse. And here’s what happens when there’s road construction narrowing a two-lane road down to one (I think the state DOTs have the message this time, it’s also still the season of winter, not construction).

TRB wrote about it, as did some academic articles. There were traffic jams reported across the country, and locations which had high levels of traffic for hours before, and especially after the event. Here’s a GIF of Google Maps traffic. A lot of the worst traffic was reported in places like rural Kentucky, Idaho and Wyoming which for all intents and purposes do not have traffic.

The eclipse passed over Wyoming, but not Colorado. So tens if not hundreds of thousands of Coloradans headed up I-25 to find totality near Casper. There are very few highways in Wyoming, so traffic was funneled onto this singular roadway. Southbound traffic from Wyoming to Colorado jumped 10-fold once the event was over. Backups were reported for the entire rest of the day, and a 4 hour drive wound up taking 10. I-25 is somewhat unique in that there are no local roadways to shoulder some of the load, so while it may not be a direct corollary to Northern New England, this level of delay may not be out of the question. For the most part, reports found traffic was densest after the event.

The weather this year may also concentrate traffic. 2017’s eclipse was mostly in the clear, but this year is going to be mostly cloudy, with only Northern New England and a swath from southern Missouri to Indianapolis in the clear. Both of these share the characteristics that they are near large population centers not in totality: the Northeast Corridor for New England (and potentially parts of upstate New York) and Chicago and Saint Louis (and others), so add to that longer-distance eclipse fans fleeing Texas and Ohio and traffic may get even worse.

Tips for optimizing for traffic? Well, first optimize for visibility. This is a couple-times-in-a-lifetime event. Once you’ve done that, consider getting yourself near the eclipse center line but pointed in the direction you want to wind up. Then get ready for a long ride home. And once I’m home, yes, I’ll scrape some data and analyze it in this space. (I might throw in some Mass Pike Superbowl data, too.)