Last week, California released its annual estimates for each city’s 2016 population growth. On the surface, the numbers look good for the state, especially for San Diego, which grew faster than the rest of California and the rest of the country.

But there is a long-term threat facing the county within its population growth trends. Rich suburbs have no more room and are experiencing low population growth. Meanwhile, the highest growth in San Diego County is in lower middle-income Vista. Viewed together, these two trends show how poor transportation and growth-restricting zoning limit the county’s access to good jobs.

The biggest cities in the state have outgrown their counties. Los Angeles grew 1.1 percent in 2016, while Los Angeles County grew just 0.6 percent. San Diego grew 1.1 percent as well, compared with only 0.9 percent for the entire county. San Jose grew 0.9 percent, compared with 0.8 percent for Santa Clara County. San Francisco is a consolidated city-county, but we can compare it with San Mateo County: San Francisco grew 1.1 percent, San Mateo County grew 0.6 percent.

In recent years, urbanist writers and advocates have posited a “back to the city” trend, in which jobs move back to city centers, cities undergo gentrification instead of white flight and central cities see significant population growth.

This is not exactly what happened in California last year.

The four largest cities in the state all posted strong population growth, but most nearby cities, relatively dense and urban ones, didn’t. In San Diego, El Cajon is actually the county’s densest city, at 6,900 people per square mile; it grew 0.5 percent. The two richest cities, Solana Beach and Del Mar, grew 0.3 percent and 0.6 percent, respectively.

The pattern is that the size of a municipality, rather than how urban it is, makes it easier to adopt pro-growth housing policies. Smaller cities that should be able to accommodate growth tend not to.

But the highest growth rate in the county is not in San Diego. The city cannot accommodate everyone who wishes to move in to take advantage of its strong job market. Outside the city proper, people move to working-class and lower middle-class suburbs and exurbs. In the past, that meant people moved to Chula Vista, which grew by 3.5 percent every year in the 2000s. It still grew by 1.1 percent in 2016, like San Diego, but is far from its past growth rate.

The frontier of San Diego County demographic growth is now in Vista, far north of central San Diego, where population grew 3 percent in 2016, assuming the state’s estimates are correct.

Vista is not poor, or excluded. It has about the same unemployment rate as San Diego, and the same average commute length. The poverty rate is somewhat higher than California’s average. Per capita income is lower, $21,764 per year compared with a county average of $31,266. It previously underwent white flight — in the 2000s, the number of non-Latino whites fell, by about 13 percent — but since 2010 its demographics have stabilized, with flat growth among whites and some growth among Hispanics.

Between 2010 and 2015, the American Community Survey has a detailed demographic breakdown of Vista’s population. The population is incrementally aging, though it is still on average several years younger than the rest of the country. What seems to be happening is a phenomenon called demographic momentum.

Vista developed relatively recently. It didn’t exist until the 1950s, and about a third of its current population moved there in the 1980s. The initial settlement involved families with children, and few older people. As a result, the city has fewer retirees than the rest of the county. Few residents have died of natural causes, while many are having children of their own. This creates lingering population growth, even if every family has exactly two children and there is no net migration. Eventually this will level off; already, Vista’s population growth since 2000 has been slower than its growth in previous decades.

Despite these factors, growth in Vista has ticked up in recent years. This indicates there has been more migration to Vista recently, on top of its demographic momentum. Vista’s growth rate increased from the 2000s to the period between 2010 and 2015, by a much larger amount than the uncertainty in the data. Then it increased again in 2016. There is a sense in which Vista represents the future of migration within San Diego County.

Judging by Vista’s demographics, this future is lower middle-class. It is not necessarily grim or poor: the declining, high-unemployment parts of the country, such as metro Detroit, are seeing net emigration, as people move elsewhere for better job opportunities. California has its own deep poverty in places like Imperial County, but it does not have the decline that Detroit and other Midwestern cities have seen.

But the future is not comfortable, either. The richest cities are not home to much population growth. San Francisco finally has strong growth, but this is after years of sluggish growth; the same is true of Los Angeles and San Diego — and in the rich suburbs, there is still very little growth. Those areas are zoned out: Their zoning allows very little replacement of smaller buildings by bigger ones. As a result, as they get more desirable, rents and property values increase, but population hardly changes.

Meanwhile, the fastest-growing major counties in the state are exurban: San Joaquin outside the Bay Area, Riverside in the Inland Empire, and Yolo and Placer in the Sacramento suburbs. Sacramento is a smaller interior metro area, and coastal demographics are less relevant to it; but San Joaquin and Riverside are both in drive-until-you-qualify territory. Within San Diego County, Vista plays the same role, even if its economic situation is somewhat different — it is poorer than San Joaquin and Riverside counties, but has far lower unemployment.

The new exurban future is also not too dependent on one industry. In its comprehensive annual financial report, Vista specifically credits its economic success to wide dispersal of jobs among many employers. It has medium-size manufacturers, as well as Wal-Mart and Target, employing a few hundred workers each, in a town with about 30,000 jobs.

In the postwar era, the middle-class suburbs, such as the Levittowns, were entirely dependent on city jobs. Men drove into the city or took the train every morning and would return in the afternoon. Suburban employment was local: government functions and some retail, but no office or manufacturing jobs. Vista is different. It is a bedroom community, but it is relatively balanced between jobs and residents, with 1.1 employed residents per job. In Riverside County, that ratio is 1.3. The highest ratios are found in outer neighborhoods within cities: Queens, in New York, is wholly dependent on Manhattan jobs and has a ratio of 1.5.

Queens shows that in the most economically integrated regions — in individual cities — there are job centers and bedroom communities. And this, ultimately, is the problem with the growth of exurbs like Vista. Vista cannot be a proper bedroom community for San Diego, since it is too far away. The transportation between Vista and the city consists of congested interstates, or of a slow, infrequent rail connection between the Sprinter and Coaster. As a result, residents of Vista mostly work locally, where there are many jobs, but that do not pay well.

This spells out a future of California growth that is not particularly exciting: exurban, lower middle-class but not impoverished, low-income but not desperate. This is likely to dampen the state’s economic growth, as more and more people are pushed away from the most productive cities.

San Diego could combat this by making sure people have access to its skilled job cluster — whether by building more housing in the city itself, or by building a better transportation network out of the rail links in the region.

If it doesn’t, it may think it’s doing fine, since a Detroit-style collapse is unlikely, but it will slowly lose its economic edge.

Alon Levy is a Paris-based mathematician and public transportation policy writer.