Here is a list of locations on state highways or ferries that Foursquare users in Washington state have used for Foursquare checkins.  If you see any that you know of that aren't on this list please let me know:

Seattle area:

I-90 Floating Bridge: http://foursquare.com/venue/67248  (claimed)

I-90 - Westbound from Issaquah - http://foursquare.com/venue/138960

i-90 eastbound: http://foursquare.com/venue/895131

520 Floating Bridge, Governor Albert D. Rosellini Bridge-Evergreen Point: http://foursquare.com/venue/206693

520 Traffic: http://foursquare.com/venue/1526431

520 West Mile Marker 6: http://foursquare.com/venue/1212986b

520/405 Interchange: http://foursquare.com/venue/1983105

520 Bike Trail: http://foursquare.com/venue/1980780

Aurora Bridge: http://foursquare.com/venue/137767

I-5 Ship Canal bridge: http://foursquare.com/venue/1055404

I-5 Express Lanes (Southbound): http://foursquare.com/venue/1171450c

I-5 Express Lanes (Northbound): http://foursquare.com/venue/2506903

I405 and I90: http://foursquare.com/venue/1377739

I-405: http://foursquare.com/venue/2481404

I-405S: http://foursquare.com/venue/1057235

i405N: http://foursquare.com/venue/1313044

I-5 between Blaine and Seattle: http://foursquare.com/venue/180075

Ferries:

The (M/V) Wenatchee: http://foursquare.com/venue/1956116

The (in)famous "I'm on a boat" badge: http://foursquare.com/user/realchase/badges/1949068

WSDOT MV Tacoma: http://foursquare.com/venue/2498930

Tacoma area:

Tacoma Narrows Bridge: http://foursquare.com/venue/733128

Tacoma Narrows Bridge Toll Plaza: http://foursquare.com/venue/1242893

Olympia Area:
I-5 North at South Tacoma Way: http://foursquare.com/venue/1953204

Rest Area:
Indian John Hill Rest Area - Eastbound - http://foursquare.com/venue/1229854

Maytown Safety Rest Area: http://foursquare.com/venue/1827242

Scatter Creek Safety Rest Area: http://foursquare.com/venue/636212

Rest Stop - I-5 at Federal Way: http://foursquare.com/venue/1467191

Park and Ride Lots:

112th & I-405 Park & Ride: http://foursquare.com/venue/879153

and many many more....

Amtrak Cascades:
amtrak cascades 513: http://foursquare.com/venue/1172897

As always, keep your focus on the road and please don't check-in while driving.