The worker actions are a bit tricky indeed. Lets see an example.
You want to build a road that demands 6 worker turns.
If you use 6 workers, then all those workers will do their job, the road will be completed and the workers will be ready to act again next round (their movement is now 0/1)
If you use 3 workers, things play out a little differently. The workers take their first turn and keep working, that is OK. Next round, you would expect those workers to finish the road and be left there with zero movement points left-just like in the previous example. This is not what happens though. The workers keep working, the road will be completed during the interturn and the workers will be available again next round.
This may confuse and give the impression that some time has been lost. It hasnt. The road gets completed at the beginning of the interturn instead of getting completed during the previous round, but this does not have significant impact because
1. In both cases the workers are ready for use next round
2. In both cases the road/mine/irrigation/railroad effect IS GATHERED, because finishing worker actions is the first thing to occur during the interturn. It may confuse you about when a project is to be finished, when a city is going to grow or when a tech is going to be researched, but there is no loss of food, shields or commerce.
3. The ONLY thing, as far as I understand, that harms you is what you describe here. You cannot use the road for transpoting units. Most of the times, we do not care about that. If a road is important to be finished during the turn due to war purposes or settler movement, we need to plan it carefuly earlier.
For this reason, we have the following weird effect. A roaded tile needs 6 workers to be railroaded. 3 workers go there and spend one round railroading. Cool. Next round, half the job is done and the 3 workers are still railroading. You can send 3 new workers there (using the existing roads), make them build railroad, the railroad is completed on the spot (like it would have been if you had used 6 workers from the start) and the 3 previous workers are immediately released from the job and are ready to move. Since you have used 3 workers and released 3 other workers, it may give the impression that you somehow won 3 worker turns, because the railroad is finished but this is not true. The only thing you gained was usage of the railroad for that round for other units.
From the above, it is very tempting to reach the conclusion that using the full number of workers needed is superior and convenient. You always know that the city growth rate/production time required/research of tech is accurate and do not need to calculate the extra boost coming from the finish of the worker actions at the beginning of the interturn. Plus, in the cases of roads, those roads can be used immediately during that turn. This is a wrong conclusion and I would really hope that this will not be the first thing you learn in the forums here. The superior strategy is to always send one worker in unroaded tiles to finish the road (or a mine/irrigation followed by a road by the same worker) unless something very weird and specific is going on at the moment that includes that tile. That is because moving a worker into an unroaded tile wastes a turn (if this does not immediately strike as easy to understand, we can explain it in more detail). So, later in the game, when you have hundreds of workers, it is convenient to move multiple of them through roads in order to irrigate something during the same turn (and so, be able to handle city screens more easily), but not while the tiles are still unroaded.
I have the suspicion that some strong players sometimes send more than 1 worker to clear jungle/wertlands, but I have not confirmed it. I guess releasing a grassland tile in 12 or 18 turns instead of 36 is something worth considering losing 1-2 worker turns....but stay away from this logic in all other cases please.