Micron’s proposal to build a large factory on Route 31 in Clay has a lot of people talking about public transit in the northern suburbs, but Onondaga County will need better planning to guide population growth so that public transit can actually work out there.

The northern suburbs can’t accommodate much more population growth with their current transportation network. These areas were almost entirely rural until pretty recently, and their road network doesn’t have the capacity for much more than a rural population. There are only a small handful of roads that cross the enormous area between Syracuse and the Oneida River, and as single-family development has converted farms to suburban sprawl where every adult makes every single trip in a car, those roads have gotten very congested. Onondaga County predicts another 4,000 homes will be built in this area in the next few years, and if every adult living in all of those new homes also makes every single trip to the grocery store, to work, to school, in a car on those same overtaxed rural roads, the traffic is going to be terrible.

Public transit can help. With fast, frequent, reliable transit service, people won’t need to use their car every time they leave the house. That gives people the option to avoid dealing with traffic, and it reduces traffic by taking cars off the road. Giving people this decent option is the only way to accommodate significant population growth without strangling the northern suburbs with car traffic.

But public transit needs population density to really work. There are a lot of different factors that influence transit ridership—the street grid, household income, building form, land use—but population density is one of the biggest. As a rough estimate, the land within walking distance of a transit stop needs at least 10,000 people per square mile in order to generate enough ridership to justify useful high frequency transit service.

Here are all of the blocks in Onondaga County with that level of population density. The overwhelming majority are in the City, and just a few are in the northern suburbs. Right now, there just aren’t enough people living near enough to any bus stop to justify high frequency bus service up there.

This isn’t a problem that goes away just by increasing the County’s population. The northern suburbs are already pretty heavily populated—way more people live in Clay than in high-bus ridership city neighborhoods like the Northside—but that population lives in sprawling suburban development that can’t support decent public transit. The map below shows the rough extent of existing residential development in the northern suburbs in black with transit supportive densities in purple. Thousands of acres have already been developed in a way that simply cannot support decent transit service. Huge yards separate neighbors from each other, apartment bans force small households into huge houses, single-use zoning makes it impossible for people to walk to neighborhood shops. This is car-only, traffic-causing development.

There’s still a lot of space up there to build homes for a lot more people, but the kinds of neighborhoods and the kinds of homes that get built in the next 50 years have to be different from those that have been built in the last 50 years if the northern suburbs are going to avoid the kind of terrible traffic that you see in sprawling cities like Atlanta. The northern suburbs need mixed-use neighborhoods where people can walk to neighborhood businesses and community institutions. They need a diverse mix of housing types like apartments and rowhouses and walkups and single-family homes of different sizes. That’s the only way to make transit work, and it’s the only way to accommodate population growth without creating terrible traffic.