I'm going to answer this question with Randonneuring in mind rather than Touring. When I'm touring, I generally just ride whatever is there and make decisions about route options on the fly.

In Randonneuring, there is a seemingly very generous 15 km/h minimum speed. "Seemingly" because that includes all rest stops, and if you're not a particularly fast cyclist, and then take a nice little lunch break in the middle of a ride, all of a sudden maintaining 15 km/h can be challenging.

When I lived in Manitoba, I never worried about the difficulty of rides because the difficulty depended on the weather rather than the terrain, and who can predict the weather? Winds that slowed me down below a 15 km/h rolling speed were rare, and there were often just enough tailwinds to push me along at a faster speed to make up for it.

When I moved to Alberta, there was more climbing, but the climbs weren't steep ... and I can do a few gradual climbs in a ride without it adversely affecting my time.

Same with Victoria (Australia).

But then we moved to Tasmania, and the climbs became frequent and steep, and all of a sudden ... I couldn't maintain that 15 km/h minimum speed. When you drop to 4 km/h going up hills, it can be hard to make that up again. But it was hard for me to judge routes ... what was a reasonably flattish route which I might be able to do, and what was too difficult?


Then about a year ago, I came across this calculation in the Road Forum:

Elevation in metres/Distance in metres * 100

So ... if a route has 500 metres of climbing over 40 kilometres, that's 500/40,000 = 0.0125 * 100 = 1.25

For me, anything under 1, is basically a flat route.
Between 1 and 1.5 is hilly, but manageable.
1.5 to 2 is quite hilly and very challenging for me. Might be touch and go whether I could finish within the 15 km/h minimum.
Over 2 ... too difficult. Out of my range.

So my route to work, if I were to cycle to work, is 275 metres over 15.2 km ... 275/15200 - 0.018 * 100 = 1.8. At this point, that's just too challenging for an early morning ride.


Anyway, that's how I calculate the difficulty of randonneuring routes, and I sometimes I use that calculation for tours and recreational rides too.