What PSI for downhill bike tires used for road biking?
You will have lower rolling resistance at a higher pressure   

A lower pressure puts more tire in contact with the ground for traction.
In the dirt this can be important.  

But you are running on the street.
Yes traction on the street is also important.
But your downhill tires are not going to have good traction on the street.    

Downhill bike and downhill tires on the street you are looking for low rolling resistance.
So pump them up to the max and no hair pin turns at 20 mph.  

If you are going to ride on the street you could mount some street type tires.
And they have tires for a mix of dirt and street.  

This is probably a duplicate some where.
I did not search.