Do I need pressure treated wood?

Assuming it will be out in the weather all the wood (not just the wood that goes into the ground) will hold up MUCH longer if it is pressure treated.

I would use poured concrete not blocks. Blocks are not that strong. Get water in them freeze & break. In truth for cost  I would use cement footing. & buy a flat bed trailer cheap to use as a bridge.  Once in place remove the tires & axels from under it. Replace boards as needed. 

Cedar is good.  It is naturally resistant to rain so if you live on the west coast where it rains almost every day then the wood will last very well.  It does not do good in the sun. No wood does.  It won't last forever...but a few years(maybe  10).

You don't NEED pressure treated wood. But it will outlast untreated many times over. It's only pennies more expensive and well worth it.

you need PT wood. Or else anti-water and termite wood like redwood or cypress. Ordinary wood will not resist rain and the resulting damp well, nor termites

Rot is not exclusive to buried applications, so I would treat the structural wood with copper napthanate if you decide against pressure treated. Especially at the cut.ends.

No it doesn’t need to be treated lumber, it could be cedar, redwood, teak, white oak, cypress or even mahogany. These all do very well exposed to the elements but pressure treated lumber is much more affordable

Where the boards touch the concrete supports would be considered ground contact.

Yes, Treated wood lasts 4 times longer outside in the weather than untreated wood.

only a fool would use untreated ..it needs to be tanalised .it looks a greeny colour ..its put into a sealed tank ..air is withdrawn ..and the fluid passes completely through the wood ...it will not rot in 20 years years as it is ..not painted 

I wouldn't use anything else, but obviously it is up to you. Most wood is fast grown and not at all durable unless pressure treated, and you still need to treat cut surfaces and ends.