Not having tried them myself I'd imagine it would very much be down to the traffic flow? If there's all sorts of cars going past, the light will not be in exactly the same place causing the trails to look a bit indistinct. From other photos I've seen it also seems to depend on the amount of additional light sources affecting the scene.
Having said all that; as Revels says, using a narrow aperture will probably reduce the glare and keep it more focused.