The Tri State Tollway has a mileage system that starts at zero (0) at both ends of the roadway. This means the mileage ascends from both the south and the north. The reason is that the Tri State is both I-294 and I-94 on the south end, and it is I-94 north of the Eden's Spur, which is Exit 25B.
Eden's Spur is the point where the two roads, both a part of the Tri State Tollway, come together.
While the road is, technically, an east-west highway, the travel directions are displayed as north and south, as that is the de facto geographic direction of the roadway through Illinois.
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The Tri State Tollway uses the following 'Class' definitions for vehicles:
Count each axle with wheels on the ground.
More detailed vehicle class information may be found on the Toll Payments page.