WELL, OF COURSE WE NEED TO MEASURE ACCESSIBILITY

MATT PETTIT
Engineer, Citilabs

Much of the recent talk around the transportation water-cooler these days has been about promoting multi-modal accessibility as a performance metric. We should no longer agonize over arbitrary highway travel times between exits A and B; rather, we need to know what destinations are reachable at and between those exits. Accessibility has been measured and defined in different capacities, most of those centering around measuring the amount of jobs accessible within a certain travel time.  (Refer to this article for more background details.) This has been a step in the right direction, however I believe the idea of transportation accessibility should instead be defined around the following question:

How can I best use the city’s transportation system to get to my daily destinations?  

While commuting to and from our jobs typically occurs during the most troublesome time to travel, shouldn’t we also be considering when and how people travel to grocery stores, hospitals or other points of interest? And if we are not considering these non-peak period trips, will accessibility to these destinations ever improve?

 

Let's Measure Accessibility

Measuring accessibility does not have to be a tedious analysis that is run out of the backroom in the planning department. Rather, measuring accessibility should be something that all planners are able to do throughout their daily workflows. In order to accomplish this, the appropriate data must be available for all to utilize. This data may include:

  • Complete roadway network information – Pedestrian and bicycle paths all the way to local and regional highway information
  • Public transportation network information – Routes, stops, and run-time information
  • Demographic data – Information about where people live and work
  • Land-Use data – Locations of hospitals, parks, schools, and grocery stores

While this data can come in different formats, geographical information systems (GIS) today allow us to interface with all of the data in a single platform.  This enables us to measure current-day accessibility and also analyze the accessibility of tomorrow — answering the question “how do we improve accessibility in our 2040 transportation plan?”

 

What's Inside That Black Box?

In order to give multi-modal transportation a fair shake in any mobility/accessibility analysis, we need to tweak the way we model individuals’ movements. The goal should be to model everyone’s potential trips. Are Traffic Analysis Zones (TAZ) really the best way we can model someone walking or biking between their home and a possible job, or even a subway station? Instead, zones should be more reflective of the types of trips we want to model. Think Parcels and Census Blocks – you get the idea. So, now that we readily have all of this information at our finger tips, what do we want to actually measure and calculate? Is calculating the number of a particular destination reachable within an arbitrary time good enough? It’s a start, however, should the destination that’s only 5 minutes away be weighted the same as  one 30 minutes away? … probably not. In addition, why don’t we simply measure accessibility with one metric that considers accessing all of our daily destinations? Accessibility scores allow us to consider access to multiple destinations (grocery stores, schools, parks, etc.), as well as assign a penalty (due to the decrease in desirability) to those destinations that are farther away. Thus, we would have one performance metric to use as a benchmark moving forward in the planning process.

Defining Accessibility within Sugar Access

Citilabs’ new accessibility tool, Sugar Access, allows users to measure and calculate accessibility metrics as defined by them, the user. Each geographic region is different, and we need to finally account for this. In the realm of mobility measurement, delay thresholds for levels of service (LOS) are constant; however, regions decide which LOS is acceptable to them. To that end, when measuring accessibility, one region may place a higher value than another region on a certain element. Having good access to arts and culture may be important in one city, while another city doesn’t give much weight to having arts and culture nearby. Wouldn’t it be great if we could account for these differing desires, and conduct analyses tailored to the nuances of each region? Well … now you can! 

Citilabs recently published a white paper discussing the methodology behind accessibility measurement. We would love your feedback about how you are currently measuring accessibility, and how it can play a critical role in planning for the future. Please add your comments below or send me a direct email to let us know what you’re doing with accessibility.

 


 

Download Accessibility Measurement White Paper



 

MATT PETTIT
Engineer, Citilabs
mpettit@citilabs.com

Through his role at Citilabs, Matt is able to empower local and regional governments to maximize the value of their transportation modeling software. Matt has played an integral role in the development of Sugar Access, Citilabs’ new multi-modal accessibility analysis tool.