led Luminous Intensity (mcd) for car headlights?

what Luminous Intensity (mcd) should be used for for car headlights, side marker/running lights, signal, and brake lights

The regulations are complicated, as you need to consider lumens of total light output, color balance of the light, and candelas humans can perceive from various directions. You'll need to check both federal and state limits.

Currently, using homemade assemblies of LEDs for headlights seems rather wasteful, as the required intensity is measured in thousands of candelas, not thousandths. In the US, your non-halogen high beams should be around 75 kcd (75,000,000 mcd) in the appropriate area of the road.

Building LED assemblies for brake lights and indicators sounds much more plausible. Check with your local Department of Transportation for the regulations.