From Seattle (3 hour drive) -

Take I-90 eastbound to Ellensburg. Continue southeast on I-82 through Yakima into the Lower Yakima Valley. Enter Prosser from I-82 at either Exit 80 (Gap Rd) or Exit 82 (Wine Country Rd).

Take I-90 West to Ritzville. Exit south on US 395 to Pasco. Exit west I-182 through Richland to I-82 junction. Exit west to Yakima (Prosser). Continue west past Benton City. Enter Prosser from I-82 at either Exit 82 (Wine Country Rd) or Exit 80 (Gap Rd).

Take I-84 eastbound to Biggs and US 97 junction. Exit US 97 North across the Columbia River and up the hill to WA SR 14 junction. THEN:

1) Right on SR 14 (eastbound, up river) to Paterson and junction with WA SR 221. Left on SR 221 North to Prosser.

OR

2) Follow US 97 North to Toppenish and junction with WA SR22. Right (east) on SR22 to Prosser.