Here’s something strange: in my experience, when people say “I have a bike,” they’re referring to a mountain bike. Ambiguity between road bike and mountain bike doesn’t exist because so often they mean the latter. None of this is strange yet. It makes sense that the abbreviated form of a word, or of groups of words, be used if the abbreviation is unlikely to cause confusion, here mountain bike = bike.
What’s strange is that most people seem to ride their mountain bikes almost exclusively on sidewalks, a far cry from the dirt and rocky terrain they were designed for. The smirk potential of the situation is enhanced by how much better suited road bikes are to riding on pavement. The difference in speed and required effort is quite staggering between the two types. I’ve only come up with a couple reasons why mountain bike dominance on roads has come to be.
#1. Price and availability: it would not surprise me to learn that the lowest priced mountain bike is much cheaper than the lowest priced road bike. Also, the recreation sections of huge department stores are likely to only stock mountain bikes. Certainly mountain bikes do better on the pavement than road bikes do in the mountains, so there’s this idea that mountain bikes will satisfy all your bike needs while road bikes can only be ridden on half the available surfaces. This selling point is moot if all the riding you’ll ever do is on the road, but don’t try talking rationally to consumerist culture. Sale! Sale!
#2. Possibly due to #1 above, most of the road bikes being ridden belong to insane biking fanatics who most noticeably distance themselves from the common Joe and common Jane by dressing in tight and oddly colored spandex garments. As such, motivation for purchase of a mountain bike may be to ensure that the rider is not perceived to be a biking fanatic. While I empathize with this motivation, I’ve found road bikes to be so much better than their counterpart at pavement riding that I’ve come up with a different solution to the biking fanatic association problem which allows for the use of a road bike: I just wear a t-shirt and jeans.