For many years it was believed that the US-Canada exchange rate was the dominant factor affecting the number of Canadians driving south to visit Washington State. When the Canadian dollar was strong in the early 1990s, border crossing were high. The Canadian dollar weakened relative to the US dollar in the mid and late 1990s, and border crossings likewise fell. However, when the Canadian dollar began to strengthen in 2003, border crossings did not increase as expected. While a number of possible reasons have been suggested, most attention has been given to increased border security in the wake of 9/11.
BPRI and Hodges, Hart, "Explaining the Decline in Border Crossings Since 1990" (2006). Border Policy Research Institute Publications. 66.