What is a reasonable speed for long distances on a bike? I am curious what a reasonable speed to travel on a bike is.  Speed will obviously vary based on the conditions in which you are riding.  I am planning on taking the GPS out with me this weekend to see how quickly I go.  Before I did that I wanted to get some benchmarks.

For the most part I will be riding an older road bike on crushed rock.  (Very small rock, with good rolling resistance but still much worse than pavement).  

I will also be riding that road bike on the road (i.e.: pavement in North America, Tarmac in Great Britain).

What is a reasonable speed on these two surfaces?  I am more interested in speed over long distances, i.e. if you were going 80 km what would your target speed be?
Speed varies widely by cyclist, depending on fitness, road conditions and traffic.  Some of my observations (cruising speed based on a flat, paved road in good condition):


20km/h (12.4 mph) - many "occasional" cyclists ride around this speed
25km/h (15.5 mph) - most commuters
30km/h (18.6 mph) - fast commuters, slower roadies
35km/h (21.7 mph) - fast roadies
any faster than that on a long flat and they're probably a racer


(based on who I pass and who passes me when riding around 30km/h)

Average speed will usually be slower than you think, once traffic stops and hills are factored in, especially over longer distances (like 80km).  On my 21km commute I'll hit 30+ on every long stretch I can, but my average still only works out to 24km/h.  For longer rides I cruise around 27-28 km/h, which is more sustainable; averaging 22-24 over a very long ride (200km) is a great pace for me.