Definite Article with Prepositional Phrases

Solution 1:

In contexts like these, use "a". Even if the hearer is familiar with the particular app, they don't know that that is the subject of the conversation until you introduce it.

The only time you might use "the" here is if a collection of which the app is a part has already been introduced in the conversation; eg

My boss let me choose which parts of our new software suite to contribute to. I'm working on the Web application to assess the current performance of traffic companies.

In that context "a" and "the" would both be possible, with "the" suggesting that the hearer already knows something about what is in the software suite, and "a" suggesting that they don't.

I'm not sure what your rule is about 'to use "the" with prepositional phrases': I don't recognise that as a rule of English at all.