As I wrote a few weeks ago, the best way for a community to accomplish anything is through consensus. History is full of "leaders" who just had to Act Now, and couldn't wait for consensus. Interestingly, many of the most dramatic abrogations of consensus involve leaders who were reacting to an existing breakdown in consensus. Think of Caesar crossing the Rubicon: there were already other Roman leaders who were operating as dictators, and he just wanted in on the act.
There have been dictators and strongmen throughout history, and there are areas where people seem to agree that consensus is not a good idea. In theory, the military operates in a top-down fashion, and there's only one captain on a ship, but in practice if you read accounts of the best military commanders and ship captains, they are constantly concerned with getting their crew to buy into the mission. If they fail, history is full of rebellious army officers and naval mutinies.
In practice, strong leaders are not always followed by other strong leaders, so any dictatorship is unsustainable. If democratic institutions promoting consensus are not in place, dictatorships tend to degenerate into corruption once the strong leader has passed from the scene.
Even when consensus-oriented institutions are in place, many people have created competing authoritarian or corrupt institutions. Corporations were one such tool: the Dutch East India Company had the right to rule its ships and colonies without the kind of consensus typically required in the home territory. These state-chartered corporations are the Renaissance ancestors to our modern authorities.
Actions taken without consensus tend to provoke a backlash, which can not only wipe out the original actions but go even further. The reactions to highway-builders like Bob Moses have played out in "freeway revolts" and the livable streets movement.
People who are expected to cooperate with the actions without first consenting can work half-heartedly, if they don't actively undermine the actions; this is probably most obvious when drivers double-park in bicycle lanes. There is also the risk that some of the people involved will not completely understand the rationale, and will thus pervert it in some way. The prime example of this last is the park-and-ride, which started out as a crutch to support transit, but became a means of allowing drivers to use long-distance transit without supporting local transit or walkable neighborhoods.