You need an ND filter to get long exposures in daylight as others have noted.
However this will probably still not give you the results you need. Long exposure shots of cars work at night time because the car head/tail lights are brighter than anything else in the scene.
During the day all you will get with a long exposure shot of cars going by is a muddy streak in front of a sharp background.
You best hope to capture some sense of movement is multiple short exposures of at most 1 second showing individual cars with some motion blur, stacked up and masked to provide the impression of a lot of cars going by.