Louis Merlin and his team at Florida Atlantic University set out to answer a deceptively simple question: what actually makes a 15-minute city work? Their answer, published in the Journal of Urban Mobility, cuts through the abstractions to reveal something concrete and actionable.
The researchers analyzed nearly 200 transit station areas across the Portland and Washington, D.C., metropolitan regions, using detailed mobility data to track thousands of real trips—not surveys, but actual movement patterns. What they discovered challenges conventional planning wisdom. Yes, proximity to grocery stores and parks matters. But the single strongest predictor of whether people meet their daily needs locally is something more fundamental: jobs.
"Neighborhoods with more jobs are far more likely to function as self-contained environments, where people live, work, shop and socialize without leaving the area," Merlin explained. The data showed that employment density stood out as the dominant factor shaping local travel behavior across both cities. In practical terms, this means that planting schools and cafes near transit stations accomplishes less than ensuring those neighborhoods actually have workplaces.
The study identified a second critical ingredient: connected street grids. Walkable, well-linked networks proved to be the strongest built-environment predictor of staying local—a finding that reinforces decades of urban design research but now with concrete evidence from observed behavior rather than theory. Together, these two factors create the conditions for genuinely livable neighborhoods.
What's striking is what walking revealed about these communities. More than 86 percent of internal trips—journeys that begin and end within the same neighborhood—happened on foot across both regions. This wasn't because people lacked options; it was because proximity works. When jobs, services, and homes are sufficiently concentrated and well-connected, people naturally choose to walk. The design of the street network and the density of employment create the conditions for this shift away from car dependency.
The research methodology itself broke new ground. Rather than relying on surveys that miss short walking trips, the team analyzed vast datasets of actual movement patterns observed through StreetLight Data. This granular approach revealed something earlier studies had obscured: the true role of employment in shaping neighborhood life. Merlin noted that previous large-scale studies of mixed-use developments had likely underestimated this effect because they focused less closely on how people actually move around transit-oriented neighborhoods.
One particularly practical finding emerged from the data: a ceiling effect. Internal trip activity plateaued at roughly 11,600 jobs per square kilometer. Beyond that threshold, adding more employment doesn't localize travel further. This gives planners a concrete target, a way to know when a neighborhood has achieved critical mass without overshooting toward density that creates other problems.
The implication for cities serious about livability is clear. If you want neighborhoods where people genuinely meet their daily needs locally—where car trips shrink, walkability flourishes, and community life concentrates on streets rather than in vehicles—focus on employment. Zoning decisions that concentrate jobs matter more than amenity checklists. The 15-minute city isn't built primarily through strategic placement of coffee shops and libraries. It's built by ensuring people can actually find work near where they live.