How many lanes in each direction are there? If there is just one lane in each direction, you will have a hard time making the case that you could not have seen the bus. If it was two or more lanes in each direction, then you might be able to make the case that your view was blocked by large vehicles. The problem with that is that you don’t seem to recall whether you just missed it or whether, in fact, your view was blocked. Moreover, it will be just your testimony against that of the cop, and the court will likely be skeptical of your testimony unless you can back it up. Certainly if you can show that much of the traffic on that road consists of large vehicles that would block the view of the bus, that would help.