Before the bridge was built , traffic had to go down into the Tarn River valley and pass along the national route N9 near the town of Millau , causing heavy congestion at the beginning and end of the July and August vacation season . Before the bridge was constructed , traffic had to descend into the Tarn River valley and pass along the route nationale N9 near the town of Millau , causing heavy congestion at the beginning and end of the July and August vacation season .