U.S. Sen. Maggie Hassan is asking for an explanation why the Canadian border remains closed to land travel to the United States.The White House has extended the current Canadian travel policy until Oct. 21 while announcing that restrictions on foreign air travel from overseas will be eased in November.Hassan was part of a group letter to President Joe Biden last week asking for fully vaccinated Canadians to be able to drive across the border before October. In a Senate Homeland Security Committee hearing Tuesday, Hassan criticized the policy, saying she doesn't understand the public health rationale.Unvaccinated Canadians can fly to the U.S. with a negative COVID-19 test, but fully vaccinated Canadians cannot enter the United States by land.Hassan said the Department of Homeland Security has an obligation to assess the economic impact of border closures on the domestic economy."Nothing like being at NASCAR in mid-July in Loudon, New Hampshire, our single biggest event of the year, and realizing that our stands were partly empty because the Canadians who usually visit and enjoy the Magic Mile couldn't come even though they had a high vaccination rate, and the public health threat wasn't explained at all, and especially in light of the fact that we were allowing Canadians to fly in, just not drive in," Hassan said.Secretary of Homeland Security Alejandro Mayorkas said the department is reviewing the impact on a daily basis and the arc of the delta variant in the United States is the primary concern.