from https://www.help.cbp.gov/s/article/Article-1253?language=en_...

> Immigration Officers, without a warrant, may "within a reasonable distance from any external boundary of the United States...board and search for non-citizens in any vessel within the territorial waters of the United States and any railcar, aircraft, conveyance, or vehicle. 8 CFR 287 (a)(1) defines reasonable distance as 100 air miles from the border.

from https://www.aclu.org/know-your-rights/border-zone

> Nearly two-thirds of the U.S. population, over 213 million people, reside within the region that CBP considers falling within the 100-mile border zone, according to the 2020 census. Most of the 10 largest cities in the U.S., such as New York City, Los Angeles, and Chicago, fall in this region. Some states, like Florida, lie entirely within this border band so their entire populations are impacted.