Given the above, some of us still love to ride hills, for both the challenge of the ascent and the thrill of the descent. I would call myself someone who loves to ride hills and have managed to do quite a number of significant hill rides over the years. I have found that I have maximum speed on hills: 50 MPH. I don't go that fast on every hill and I will never pedal madly to achieve a high speed on a downhill. And I don't think that speed is safe - a tire blowout could be impossible to handle safely. Now that I am bit older and have children, I am more risk averse and I may not hit 50 MPH again, or even 40MPH.
But speed alone doesn't tell you the risk of a crash. My maximum speed depends on traffic, road conditions, how straight the road is, how tight the turns are, how wet the road is, and how mortal I am feeling. I realize that racers exceed my maximum speed and do it in races with other riders in close proximity but I am not a racer. Riding is my pleasure not my business. I will try to give a hint of how I might assess a road and how fast I may ride by way of a few examples. Links are to either maps.google.com or gmap-pedometer.com, which is handy for looking a elevation profiles.
I did this ride in the summer of 2001 on my 1997 Lemond Alpe d'Huez. There were two significant climbs and descents on this ride, Franconia Notch and Kinsman Notch and the descents could not have felt more different. The descent of Franconia Notch north into the town of Franconia was harrowing between miles 12 and 13, a very steep pitch on a narrow section of road with significant curves, and there was traffic on the road. I was holding on with white knuckles. The road was in rough shape at the time and I braked at 45 MPH trying to slow myself down - I checked my max speed after the descent; I didn't have the leisure to check the computer in those conditions. It was one of the scariest descents I have experienced. Now look at the descent of the east side of Kinsman Notch, around 34.5 miles into the ride. The road was wide, freshly paved, more or less straight, had a wide shoulder, and was empty. I started the descent and quickly accelerated but felt that I wasn't going all that fast. I took a quick look at my computer and saw that my current speed was 48 MPH. I was amazed. I went from terrified at 45MPH to completely relaxed at 48 MPH. The primary difference is that one road was in bad shape, had traffic, and had a significant turn and the other road was in great shape and empty. In retrospect, if I would have braked much earlier on the descent into Franconia I would have been far happier. High speed didn't prove anything and in that case it could have been my undoing. Here is the Google Street view of Kinsman Notch looking at the top of the descent.
I did a descent similar to the east side of Kinsman on the 2009 100K D2R2 on my 2009 Surly Cross with 700x32 tires. The descent into Colrain is about a mile long and loses about 450 feet of elevation. The road had curves but were quite gradual, the road surface was smooth, there was a decent shoulder, and the cars were few and their drivers were very polite. I watched my computer on this descent as my speed increased from 30 to 35 to 40 to 45 to 50 MPH. It was a slow acceleration but the road was long enough for me to attain my highest speed I achieved on a bicycle. The handling of the Surly, with its reasonably long wheelbase and the wide tires made the descent remarkably safe feeling. There were several other cyclist on the road with me but we were well spaced for a safe descent. I might be tempted to let go on that road again, given good conditions.
I did a cool ride in northwest Massachusetts in the summer of 2001, again on my 1997 Lemond Alpe d'Huez. The ride starts in Ashfield, MA, where some friends of mine live, and traverses some hilly, beautiful, and mostly empty country. The significant descent is from Hawley to Charlemont on East Hawley Road. There is a hairpin turn about 2.5 miles into the descent. The road turns sharply left and straight ahead is a gravel driveway. The turn comes suddenly and a bike with 23 or 25mm tires would likely not survive the driveway at high speeds. When I tried to signal the upcoming turn to my riding partner, I didn't brake and increased my speed to the 40+ MPH range. I slowed to a stop rather than trying to make the turn, which would have been next to impossible. The next turn, to the right a little over 3 miles into the descent, is quite difficult as well. Again, I signaled the upcoming turn to my companion and gained speed. The turn at approximately 45 MPH was difficult physically. The momentum of my bike was overpowering my ability to turn and avoid running off the road. With some braking I was able to make the turn. I did this descent one time earlier, on my own, and did not find it quite so scary. Signaling was my main problem on this ride. At 40+ MPH you travel pretty far in 4 or 5 seconds - close to 300 feet, that's 300 feet closer to a hard turn you have to make. Having good maps and having a plan before a descent is the best idea to avoid this.
Winds issues can influence your stability on a bike, especially on a descent. On this ride, on my 2007 Independent Fabrication Club Racer, the descent to the Deerfield River valley on Route 9, west of Wilmington, VT is not overwhelmingly fast but is considerable, about 700 feet in 1.5 miles. What stood out at the time was trying to stabilize my bike at the top of the descent in a strong wind. I did what I should have been doing earlier, clamping my seat with my thighs and my top tube with my knees, with my body back a couple of inches from my usual riding position. Once locked in, my ride stabilized and I was off enjoying the ride downhill. At the time I attributed the instability to the bike design, which was described by the designer as a refined stage racer, responding easily to body movement. The usual Club Racer design was shortened a bit while retaining room for 700x25 tires with fenders and 700x28 tires without. I thought the shorter design was the root cause of this which was a bummer. I have not noticed this since and I now think the wind was indeed the culprit.
Some other notable descents:
Hurricane Mountain Road in North Conway, NH - winding, steep, you will use your brakes!
Route 143 east of Worthington, MA - the curve at mile 1 is intense at high speeds, use caution here.
Obstruction Point Road to Hurricane Ridge to Port Angeles, WA - I rode up to Obstruction Point from the campground (at around 1800 feet) and back on one day and did the last of the descent with my panniers on the second day.
Monarch Pass heading east on US 50 - not that big a drop (1000 feet in 3 miles) but I did it after sunset on a fully loaded touring bike.
Don't take my word on what your maximum speed should be. You should be doing what is comfortable at the time, based on the road conditions, wind, and the rest of what I described. There is no winning on hills, only enjoyment of the climb and the descent. Who cares if you don't set a new land speed record on your way down? Survival in one piece is my goal. And now that I have kids and am finally growing up, my speed is likely to be less than what I talked about here. YMMV.