March 2, 2016

Interactive visualization: How Seattle commuters link transportation modes

How people commute to work is an important question for cities grappling with congestion, air pollution and other problems caused by growing numbers of vehicles. Motivating commuters to shift from personal cars to other modes is easiest when we first understand what people are currently doing and why. Here we present a visualization of how commuters in Seattle link multiple modes within their commutes.

Sankey Diagrams show the key sources and ultimate dispositions of a flow. They have been applied to everything from materials to energy to Napoleon’s disastrous march on Moscow. But as far as we know, they have not previously been used to visualize multimodal travel behavior.

Here, we construct a Sankey diagram to visualize linkages of commute modes reported in the 2014 Puget Sound Regional travel survey.  The set of rectangles on the left-hand side represents the transportation modes commuters used when leaving their homes, with the height of the rectangle indicating the proportion of people using that mode. The connecting bands indicate how many people switched from that mode to each of the modes indicated in the second set of rectangles, or continued all the way to work (the rectangle on the far right). In the Sankey diagram below, you can place your cursor over each flow in order to highlight it. (If the interactive tool isn’t working for you, there are static versions at the bottom of the page).

The diagram shows that a large portion, around 65%, drove or carpooled to work, and for most of these people the car is their only mode. However, a few drive before transferring to the bus or other modes.

To make it easier to see the flows of the multi-modal commuters, we created another version of the diagram that omits the people who drive or carpool directly to work:

The next most popular commute modes are bus and walk. Most people who use the bus as their first mode end up taking it all the way to work, but a significant number of people drive or walk for their first leg before continuing by bus. Most bicycle commuters go from home to work on a bike, and very few use a bike before or after another mode. Overall, 90% percent of the survey respondents use just one mode to travel to work while the other 10% use a combination of modes. Less than 5% of surveyed population switched modes three times or more during their trip to work.

Based on the available transportation options, 35% of people are using modes other than driving personal cars in the Puget Sound region. This is much better than the national average of 10%, but there is still plenty of room for improvement. As an example, park & ride facilities are a key enabler of multimodal trips (i.e. drive to bus), but nearly half of King County Metro’s park & ride lots (accounting for 56% of their parking spaces) are already at capacity. Also, from our diagram we can see that transferring modes from biking to bus is not common. This could be due to environmental conditions such as weather and hilly topography making biking undesirable for commuters in general, although this would not explain why some people do choose to bike directly to work. An alternative explanation is that there may be inadequate facilities to support biking to transit, such as insufficient numbers of bike lockers or inconvenient loading of bikes onto buses.

Visualizing multimodal commutes cannot answer all our questions, but it can help us to better understand how people are currently linking modes and to generate hypotheses to explain this behavior.