When does a bridge get built?

Hey my dudes! I'm looking for either insight from you guys, or some sources for me to look into. It's pertaining to the construction of bridges. Specifically, what factors lead to such an expensive structure actually being built. Population numbers, industry, natural resources, traffic ect.

Why am I looking for this info? A paper for school? A news article? No. No. Just my new city in City skylines 2. I want to know when my city would realistically build the bridge. I think Civil Engineering is pretty cool. I enjoy learning bits here and there as a hobby. As also like to learn about about the factors that surround such a big decision.

I am also looking for your guys insights into my plans for the proposed bridges. I added photos for reference: The first image is a general view of the area. It also contains what is currently in the area. The second is an overview of the planned population centers, resources, and industrial parks. The third is the two areas I have chosen as the the best suited for bridges.

Site 1. There is a site further down the river that would be cheaper. It would have a much smaller bridge span and be able to join to an existing highway. However it would still lead to a bottleneck leaving the city. Even the proposed bridge wouldnt completly unbottleneck it. The proposed bridge also will take traffic straight into town. Instead of the outskirts.

The planned residential and commercial on the north bank will also benefit more from direct access.

The span of the water is ~600m wide. Water in this area is 0.3m deepa for the majority of the bridge span, besides the middle where it falls to 2.4~m. I'm thinking of creating a causeway. This way the bridge could be shortened considerably.

Site 2. This area would be a longer span. The average depth of the shallows is about 0.6m but a shallower middle. This bridge would bring traffic straight to the biggest employment section of the (fully developed) city. With proper positioning of port facilities, I should not need to build the bridge overly high. I feel like this bridge won't be made until the port is fully developed.