(I did not find a roundabout, but highway was easy to approach from the bridge and parking spot is like 50 metres far. - Solop, Dec 2016)
*The bridge mentioned may actually be the bridge over the railroad tracks. If you get off the bus at the stop after going over the bridge and walk back you can find a roundabout whose left path does indeed leads to a second roundabout whose left path leads to the motorway. However, if you are headed towards Kosovo it might be easier to ride to the last stop and then hitch/walk about 6 kilometers further as this road merges with the actual road leading to Kosovo and is fairly easy to hitch on because it isn't officially a motorway.
Another possibility is to take the train towards Doljevac from the main railway station. Get out at Doljevac station (10 -15 minutes ride, ticket price 64 RSD, about 30 RSD if you manage to bribe the conductor), and take a 10 minutes walk to the motorway toll station (accros the fields).