Which county routes run mostly one way

A county page shows a net figure, and a net figure hides how much traffic runs against it. This note looks at single routes instead. For each pair of counties where the IRS lists a flow in both directions, it divides the larger count by the smaller one. A ratio of 1.0 means the two directions matched exactly. A ratio of 3.0 means three returns went one way for every return that came back.

The first table keeps only routes with at least 300 returns on the busier side, so a handful of households cannot produce an extreme value. Suffolk County, NY to Horry County, SC leads, with 320 returns against 50 coming back across 596 miles. Several of the others run from the boroughs of New York City into New Jersey, Pennsylvania, or Connecticut, or from Miami-Dade and Broward to other Florida counties.

Balance is the normal state

Across all 21,750 county pairs listed both ways in 2022–23, 46% had a ratio of 1.25 or less. Only 9.5% reached 2.0, and 1.4% reached 3.0. Among the busier pairs the pattern holds: 200 of 1,920 reached 2.0 and 38 reached 3.0.

That result has a limit worth stating. Another 10,114 listed routes have no listed flow coming back, because the return direction stayed under the 20-return threshold. Their true ratio cannot be calculated. The median one carries only 24 returns, so a one-way reading would be guesswork, and they are left out of everything here.

Some large routes come very close to even. San Mateo County, CA and Santa Clara County, CA exchanged 3,874 and 3,879 returns. Distance does not rule it out either: San Diego County, CA and King County, WA, 1,034 miles apart, traded 1,128 and 1,112.

Who sends and who receives

Counting the 200 busier routes with a ratio of 2.0 or more shows where the imbalance gathers. Bronx County, NY is the sending side on 19 of them, followed by Miami-Dade County, FL with 16, Queens County, NY with 14, Broward County, FL with 11 and Kings County, NY with 10. The receiving side is more scattered. New York County, NY tops it with 10 routes, and Palm Beach County, FL follows with 7.

Manhattan on the receiving end may look surprising next to its Bronx and Queens neighbors on the sending end. The note on where people leaving the biggest counties go gives the wider picture for large counties, and the route figures here add one detail: a county can send heavily along some routes while drawing heavily along others.

Longer routes lean more

Imbalance rises with distance, though not by much. For busy routes of 50 miles or less the median ratio is 1.29; for those over 500 miles it is 1.48. The data cannot say why any one household moved, so the table describes the pattern and stops there.

State lines make a difference of a similar size. Among the busy pairs, the 1,409 inside one state have a median ratio of 1.30, and the 511 that cross a state line have 1.48. Part of that overlaps with distance, since a route across a state line tends to be longer.

The busier direction also tends to point somewhere specific. Of the 200 routes with a ratio of 2.0 or more where both counties have complete totals, 188 ran toward the county with the higher overall net migration rate, or 94%. So a lopsided route usually agrees with the broader balance of the two counties, though in the remaining 12 most of the traffic went toward the county with the lower rate.

A large net is not the same as a lopsided route

The routes with the largest net flows are nearly all short metro routes with heavy traffic both ways. Los Angeles County, CA to San Bernardino County, CA had the largest net of any route, 7,438 returns, yet its ratio is only 1.7. None of the ten largest exceeds 2.2. The first table and the last one share no route.

So the two rankings answer different questions. A net count shows how many households one county gained from another. A ratio shows how little traffic came back. Small differences deserve care either way, since for 2022–23 the agency changed its matching method. Definitions are on the method page.

The 100 busiest routes are listed with their return flows, and the two-county comparison shows both directions for any pair over four filing years.

Sources

Source: IRS Statistics of Income, SOI Tax Stats – Migration Data (county-to-county and state-to-state files, 2019–20 to 2022–23; latest release used: 2022–23), retrieved October 28, 2026. County locations: US Census Bureau, 2023 Gazetteer Files. Both are US government works in the public domain; calculations by countymoves, explained in How to read the data.

Original files: irs.gov/statistics/soi-tax-stats-migration-data.