I have seen creative examples of cars at night taken with a slow shutter speed.
Image
And I tried to replicate this during the day. I have tried everything I can:
Smallest aperture ever, lowest ISO and exposure to -5. The shutter is set for 30 seconds. What I ended up with is a white picture. Plain white.
You can't get the same picture during day. At night the tail-lights and headlights are very very much brighter than the background and the roadway, so they stand out. The difference will measured in several, if not many, stops difference. During the day the light emitted by the headlights and tail-lights is overpowered by the light from the sun. While there will be a very little bit more light from the headlights than the sunlight reflected back from the background and roadway, it will be measured in a small fraction of a stop of light, likely.
If you are after the look of blurring cars, then that is different. The exposure necessary to do that is entirely dependent on many factors, including the lense focal length and the length of the roadway visible in the viewfinder (which is tied into the distance you are from the roadway.)
One of the complicating factors is that if you extend the exposure time too much, you may not see blurry cars at all... You may make them disappear entirely. The rule of thumb is that (more or less) a object has to be (more or less) stationary for about half the time of the exposure to register. So in a 30 sec exposure of a road, a car would need to occupy the same space for at least 15 sec (more or less) to register. If you want blurring, then the car can move a bit such that the majority of the car moves through the same space for the exposure. The parts of the car that were moving towards or away from that shared space form the transparent blur.
If a car on a roadway is in your view finder for a second, and fills the viewfinder (more or less) then your exposure times should be in the 1/2 second range, give or take 2 or 3 stops.
Bright lights (from headlights and tail-lights at night, for example) are exception because they are emitting light - not reflecting the ambient light.
If only there was 0.005 ISO, but that isn't an option. The lowest can only go up to 100.
Once upon a time in photo school I had a chance to use a film, Kodak - I believe, that (depending on your development) could hit ISO .001. It was pretty cool to use.
...
30 seconds is a must, any lower and the effect will not be seen.
At 30 seconds you are making the cars disappear entirely.
Did you ever try this? I didn't know it would be this hard.
And this is why professional photographers get paid... And to teach, of course...
The exposure actually resulted into a darker image, I tried this with an average settings, it doesn't make the picture any whiter. So wen I made it -5, It was helping me. But the shutter isn't.
When you have set your camera to the "darkest" setting, changing the exposure compensation dial won't do anything. You can't make the camera go past its limits. All the exposure compensation dial does is alter the sensitivity of the light meter. If the meter should read 100th of a second at f/4, then changing the compensation dial will merely tell the meter to report a different setting. Or of you are using Av or Tv to report to itself the altered reading.
Good Luck. You sound keen...