Who wrote the notice and to whom is it addressed (e.g. a person in authority, addressing it to a general public? This might explain what appears to be a rather abrupt, ellipted imperative: ‘Don't ride/park your bicycle here!’).

How do we know what it means? In fact, in the situation it was taken from (the window of a bicycle-hire shop), it meant ‘We have no more bicycles left to hire out’. The notice was displayed at the high season for bicycle hire, and the most plausible interpretation was that the shop was informing potential customers that it had run out of bicycles. So the grammar was not an imperative, but a statement. What factors enable us to interpret this? They are clearly not ‘in’ the text, but are an interpretation based on the text in its context.