Nairobi - Mai Mahiu - Narok - Mau Narok Njoro - Nakuru

Another alternative would be Mai Mahiu - Narok - Mau Narok - Njoro - Nakuru. It’s significantly longer, 266Km compared to A104 at 160Km, but it barely has traffic for the most part. It doesn’t merge with A104 until past Nakuru. The downside is the Mai Mahiu Escarpment section that’s wildly unpredictable. If you can somehow sneak past that then the rest is breezy.