If you've got to cut a very long piece of wood like this to a narrower width but you don't have a table saw, well don't worry, you can do a very good job with a circular saw if you'll follow these suggestions.
Now this happens to be pressure treated lumber. It has a very high moisture content. So for that you might want to select a blade that's designed especially for pressure treated lumber. It has a special non-stick coating that reduces friction as the blade is cutting through the wet wood.
Next, fit your circular saw with an edge guide like this one right here. It slides along the edge of the lumber giving you a uniform cut from one end to the next. Third, be sure the blade depth is set so that the blade just goes barely through the wood, maybe about a quarter of an inch. And then put scrap pieces of lumber on the sawhorse so that you can cut right through. This way, you're all set to make a cut from one end of the board, all the way to the next just like this.
Using the right blade and the right setup will turn any long rip into a zip.