Last week, I posted some of Metro’s annual ridership data on Twitter, and Carter Rubin noted that Expo outperformed the Blue Line in boardings per mile in 2017. The Expo Line averaged 4,000 boardings per mile compared to 3,200 for the Blue Line. Per-mile ridership is a measure of a transit route’s efficiency – as trains get fuller, more service is provided at the same operational cost.

Among Metro’s light rail lines, Expo is now – and probably will remain – tops in this particular metric. The Blue Line did at one point average 4,000 boardings per mile, which, given its longer route length, equated to 88,000 boardings per day. But rapidly deteriorating service on the Los Angeles-to-Long Beach route has seen the Blue Line shed about 15,000 weekday riders since then, with no evident relief in the near future.

Expo’s success is most pronounced in comparison to other lines in Los Angeles. 4,000 boardings per mile is not exceptional for light rail. Even in the U.S., Boston’s Green Line system averages 2.5 times that number. For that reason, I said I would be more impressed if Expo were achieving at least the per-mile ridership of the Red Line subway system, about 8,000 per mile, or double the weekday riders it carries today. Carter’s response was skeptical, and got me thinking about this a bit more. Could the Expo Line carry 120,000 daily passengers? And what would it look like for it to do so?

To put it another way: Given Metro’s operating standards, and before a major capital investment (like a new vehicle fleet, lengthening of platforms, or new rail infrastructure) becomes necessary, does the current Expo Line have enough available space to double its passenger volume?

Metro allows for a full load of 164 passengers in each light rail car, and 218 passengers per car as a maximum crush load. Expo trains are composed of three cars each, and, when the Regional Connector opens in 2021, 12 trains will be able to run per hour. Using these numbers, the Expo Line could handle up to 7,848 passengers per hour passing through a given point in a given direction.

In practical usage, crowdedness fluctuates by time of day, direction of travel, and location along the Expo Line, which was designed to handle the travel volumes planners expected during peak-hour, peak-directional travel. So, functionally, it’s trivial to rule out the upper bound you’d get if every train ran full through the busiest point for the entire service day [7,848 passengers/hr x 22 hrs x 2 directions = 345,312 passengers/day; 23,000 boardings per mile]. And, if Expo ran out of space, it’s likely that it would be while only being really busy for a small portion of the service day.

Since we’re dealing with not only number of boardings, but also when and where those boardings are taking place, it will help to separate Expo riders into the two main rush hour commuting patterns, and a third group who ride during off-peak hours. The 2017 budget year, which ended in June, is the most recent for which Metro has made directional travel data available, and Expo’s ridership during the period broke down as follows.

Travel Pattern Boardings Eastbound AM/ Westbound PM 10,640 Westbound AM/ Eastbound PM 17,798 Off-Peak 23,539 Total Boardings 51,976

About 55% of ridership on the Expo Line occurs during peak hours, basically tracking the systemwide average of 58%. The westbound commute is stronger than the eastbound commute, and so we’ll focus there.

The morning rush has higher per-train loads than the evening rush, despite greater passenger volumes in the evening. Morning activity is more concentrated around the 7:00 AM hour, whereas it’s more evenly distributed during the PM rush.

These graphs show the extent to which available space on Expo trains is being taken up by passengers when the line is at its busiest. Certainly, it doesn’t look like Expo is on the verge of running out of space. So let’s assume the line hits 120,000 daily boardings, while maintaining the proportion of riders commuting in either direction:

Travel Pattern Boardings Eastbound AM / Westbound PM 24,564 Westbound AM / Eastbound PM 41,091 Off-Peak 54,345 Total Boardings 120,000

And, if we assume those additional riders use the system the same way it’s being used today, the increased passenger load would look like this:

If we get to this point, then we’re brushing up against a full load average during the 7AM hour. But that average covers 12 trains per day for hundreds of weekdays throughout the year, and so it’s probably concealing a lot of individual crush-load trains. And a lot of crush-load trains means a lot of vocally-displeased passengers.

While the tolerance for crowding in Los Angeles seems to be lower than elsewhere, it’s worth noting that riders in general tend to interact differently with crowded train lines than with non-crowded lines.

Passengers will avoid crowding if they can. That doesn’t mean they’ll stop riding (at least not as a first resort), but rather that they’ll seek out more comfortable, less irritating ways to take transit. Some will adjust the time of their trip to be earlier or later in the rush hour period, or to be outside of it altogether. Some will change where they board in order to get upstream and board at less crowded points. Behaviors like these serve to spread crowding more evenly throughout the line and across service hours. So in our example we might expect the 8AM and 6AM hours to become busier to the extent that they offer relief, and passenger loads to increase further and further east of Baldwin Hills.

This is only one example of how Expo could look if the number of passengers it carries doubles. We’ve assumed that passenger behavior will stay the same, when it almost certainly will not. In coming years, the openings of the Crenshaw Line, Regional Connector, Purple Line, and (distantly) the Sepulveda Line will each reshape Expo in ways we can guess at, but can’t be sure of.

As far as personal speculation goes, it seems to me impossible that the Expo Line could ever double its off-peak ridership without a thorough reconsideration of Metro’s organizational attitude about off-peak service. Trains that run five times an hour midday are not that useful, and potential riders will probably never opt for them, even though they are still making trips at those hours. So, in all likelihood, if Expo is ever going to make it to 120,000 daily riders, we’d have to see much larger numbers of westsiders using the train either to commute downtown or for travel within the westside.

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