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When more people live near a trolley station, more people use the trolley.
It’s a logical relationship and it’s considered a given in planning and development circles (and it’s been confirmed in academic research): To increase public transportation usage, increase the population density right around stations.
And sure enough, it’s true of San Diego’s trolley system, too.
There’s a strong correlation between daily activity at trolley stations and the number of people who live within a half mile of each station, according to ridership data provided by the trolley operator, the Metropolitan Transit System and population density numbers pulled from the 2010 census.
Here are those numbers. Each blue dot is a trolley station. The farther to the right a dot is, the larger the population in the half-mile surrounding it; the higher up it is, the greater its daily activity level. The trendline shows that in general, as population density increases, so does ridership.
This analysis removed some outlier stations. The ridership numbers from MTS count every person who gets on or off of a trolley car. That means when people transfer from one trolley line to another, they’re counted as additional activity at that station, even though they didn’t come from or go to the neighborhood around the station.
To get a clearer picture, we removed stations that are primarily used as transfers, not destinations. We also removed extreme outliers on the low end (Civic Center) and the high end (San Ysidro) to get a clearer picture of typical station activity.
The result for all the remaining stations is clear: Density has a lot to do with how many people ride the trolley. Advocating for more trolley use and opposing increased density are contradictory positions.
That might seem like common sense. And in fact, the city of San Diego has already codified the position – in its general plan – that future development should happen in dense clusters near transit stations in order to boost public transportation usage, drive down the number of cars on local roads and decrease the city’s overall greenhouse gas emissions.
In practice, that hasn’t gone so well.
When those sorts of developments are actually proposed, people who live in the area affected aren’t usually thrilled about the prospect of increased density, and the parking and traffic concerns that come with it.
Often, those residents are wary of the idea that density spurs ridership.
But the data mostly proves developers and city planners right. It turns out the relationship between building directly around a trolley station and people actually deciding to use the trolley isn’t just conventional wisdom.
For those interested, here’s the trolley activity and population density data for each station.
Daniel Sheehan, geographer and GIS analyst with the Built Environment and Health Project (beh.columbia.edu) at Columbia University’s Department of Epidemiology, determined the population density within a half mile of each trolley station based on 2010 census data.