Maybe you have a modded file (ped.dat, but i am not sure) that spawns the motorcycles, but not the peds riding it?
The only ped mods I am aware of are ones I made myself, simple tweaks here and there. I don't recall any other ped mods. The only thing that comes to mind is the BC7 mod, but I dont think that applies here.
They literally pop out of nowhere, essentially falling from the air on the horizon so that by the time I actually get to it, several cars have already collided as they slam their brakes on. This naturally causes a pile up that I have to swerve to avoid if I am lucky. It seems particular to interstates where the traffic is moving fast, or at least I am.
Occasionally it will happen where there arent any cars around to slam their brakes on, and you can see it in the distance just skidding along on the ground coming to a rest. Along the Pacific Coast Highway I have even noticed that once I skidded off the road myself into the water, and a motorcycle spawned into the water right above me. To be sure, I am not hitting any spawn functions.
Perhaps it is my wireless keyboard, I dont know. Except if it was my keyboard faking a spawn function through a signal error, the only key function would be through BC7, except it is always motorcycles, not the last vehicle I deliberately spawned.
So I assumed it was just a game feature to add challenge to high speed driving. Except it happens so often as to detract from realism.
Speaking of realism, I noticed a while back that the BlackJack tables dont randomly select hands. I played enough marathon sessions of BlackJack in Venturas to notice that the exact same dealer/player hand combinations were recurring. It was disappointing; I thought I was playing a truly random deck. But often I began to notice that if I had a certain hand by suit and number, the dealer had a recurring hand to match the game exactly. It got to be where I knew what he was going to deal himself by what I was holding. I could decide what I would do of course, but the deck was literally already stacked. Apparently the game treats BJack as individual sets of controlled variables, not as a random generator.
Sorry for prattling. I haven't been here in a while.