through表示“贯通、直穿、透过、穿过”的意思,即是从一头(边)贯穿到另一头(边).
例The river runs through our city.这条河流经我们市.
He passed through the hall.他穿过大厅
across表示“横穿、横过、横渡、横跨”
例:I swam across the Changjiang River 20 years ago.20年前我横渡了长江.
Look left and right before you go across the street.过马路时要左右看.
而这里车是在公路上直行,所以用through