1

I would like to express the importance of a surveillance system in preventing urban disasters. First I present some facts about a certain tragic event, then I claim that a surveillance mechanism could have helped us prevent such a disaster and save lives.

Here goes my sentence:

An effective surveillance mechanism could have alarmed us take necessary actions and eventually save those people.

but what I really wanted to was to stress that this mechanism has the ability to sense the overcrowding. I would like to say that it could have alarmed us against overcrowding. So the kind of sentence that I intend to build is closer to following in terms of content:

An effective surveillance mechanism could have alarmed us against overcrowding, helped us take necessary actions and save those people.

But but I cannot find a way to formulate this sentence in a grammatically correct way.

I would appreaciate any help on constructing such a sentence.

3

Your problem isn't the structure of your sentence, but rather that you misunderstand the use of "alarm" as a verb:

alarm (v):
1. Make (someone) feel frightened, disturbed, or in danger.
2. (be alarmed) Be fitted or protected with an alarm.

With the first definition, when you "alarm" someone you make them upset and fearful. Example:

Susan was alarmed by her friend's erratic driving, and begged her to pull over.

The second definition is almost exclusively used as the past participle, and in a very limited context that doesn't fit your sentence.

Instead David Siegel's suggestion of "alert" makes more sense.

alert (v): Warn (someone) of a danger or problem.

to put it another way: An alarm is designed to alert someone that something is wrong. You can set up alarms that detect overcrowding, but the action of these alarms is to raise awareness:

An effective urban surveillance system could have alerted authorities to potentially dangerous overcrowding, giving them time to take steps to prevent injury or death.

And precede or follow this with the description of such a system:

The system includes a network of sensors that measure crowd density, along with various alarms triggered when density reaches dangerous levels.

3

The phrase "could have alarmed us take necessary actions' is not grammatical. one could write "could have alarmed us to take necessary actions", but much better would be "could have alerted us to take necessary actions". See this google ngram

Similarly while "could have alarmed us against overcrowding" is not incorrect, "could have alerted us against overcrowding" (no "to" is needed in this construction) is much more natural. See this ngram chart

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