I need to take the photo of the trailing vehicles on road at day time. I tried a shot in bulb mode at 100 ISO and f/22 of a road in daytime. But that photo is too bight(link below). How can i get the trailing vehicles at daytime?
What you are probably looking for is a 10-stop ND filter. Lee and Hitech make large square filters - Lee calls theirs the "big stopper". B+W make a screw-in version that is less expensive.
These will roughly allow for 1000 times the exposure. So instead of 1/250th of a second, you can expose for 1000 * 1/250 = 4 seconds.
If you want even longer exposures, best to shoot in early morning or early evening when it's not as bright - where 1/60 exposure would become 15-20 seconds.
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.
You have to limit incoming light even more, since it is daylight.
You can achieve this by stacking (putting on) multiple Natural Density filters. You might try to get Cokin or Lee filter holders and buy extra set with couple ND filters and try how many filters you have to use to stop enough light. Those filters comes as ND2, ND4 etc... depending on how much light block.
Also Polarizing filter blocks around 1-3 stops which will be too little in your case, and also adds polarizing effect which you might not want.
As mentioned, you would need to consider using an ND filter. The LEE big stopper is a well respected filter and should give the results you are looking for. Here is a link to the Lee Website
The cokin ND filters are known to give a magenta colour cast when used, which, unless you like that effect on a specific image, you would have to fix in post processing.