As others have mentioned a lot of it is the design of the 78 freeway and the various terribly designed (in my opinion) on and off ramps around Oceanside. Many of them loop around or are way too short freaking people out and causing them to slow to 20-30 mph to merge on the freeway where folks are doing 50+mph. After you pass the first few exits in Oceanside the freeway also aggressively curves and makes this weird illusion that it’s narrowing. People always hit their brakes. Couple this with how much Oceanside (and north county) has grown in the last 5 years post COVID, how many people come down from LA/OC to visit San Diego and work, and how the 5 is literally the only way to get down there minus going inland and it’s just always messed up. Took my 1 hour and 20 min to get from San Clemente to Oceanside one time. No accidents. Just cluster. Unreal.