going fast doesn't earn you brownie points. Go at speed you are comfortable. As for using the speed to carry you over the next hill, it is only good for hills that we used to call "ooped-ee-doo" (however its spelled) generally hills of 100 feet in height or less. anything more and the speed you carry on downhill gets lost beyond the first 100 vertical feet. (just a rule of thumb).

If you are worried about downhill speed, and want to increase it, read oakleaf's explanation carefully. At times, I was looking 2/10 of mile ahead of me (12seconds), scanning little to the left little to the right at 2/10 mile ahead to see what the danger might be (even though 2/10 mile was no where near what I needed to come to a complete stop). Scrub/bleed off/slow down before you enter a turn. Learn to make a turn properly. Technique can make a difference at speed through a turn. Ride within your ability. And gradually learn to expand your ability. There is nothing to be ashamed at slowly coming down a hill.

I prefer to ride with women just for this reason. We are more prudent and careful.