She always travels by road. Is it correct to use the preposition by here because we use by normally with the means of transportation like She always travels by bus.?
When discussing modes of transport - road, rail, air, land, sea, river, canal, etc, we use 'by'.
In the UK, it is cheaper to travel by bus than by train. Travelling by air is usually more expensive, but quicker.
If we are discussing the type of vehicle or craft, we can use 'on' or 'in' before the word for the conveyance: a car, boat, ship, plane, bus.
I am going to London in a car tomorrow. My mother came to my town on a train.
Prepositions can be very ... flexible. You can say "travel by X" where X is a kind of vehicle, like "travel by bus", "travel by plane", etc. You can also say "travel by X" where X is the "medium" you travel through, like "travel by road", "travel by air", "travel by sea".
For that matter you could use "by" in a totally different sense, like "She always travels by herself".
They're all valid. Yes, clearly "by" does not mean the same thing in all these cases.