Hello,
Tim ...
It's very interesting how having ALL the data makes a BIG difference in determining what really might have happened. (Someday I may learn NOT to jump to conclusions!) ...

Knowing that the taxi was only going about 5 to 10 miles per hour is probably one of the most important pieces of information in this investigation.
The length of the skid mark reveals that the motorcycle was traveling at approx 40 mph, impact speed was approx 30 to 35 mph.
This would mean that the tractor may have been actually traveling faster that the taxi. If the tractor was closing on the taxi, then it may indeed have been very difficult for the motorcyclist to see the taxi.
You have never said what the speed limit was for this particular stretch of highway. However, for most roads, 40MPH does not seem excessive. Therefore, it seems reasonable (if there was no oncoming traffic) that the driver of the motorcycle would want to pass a slow moving tractor. (Unfortunately, there seems to have been a taxi in front of the tractor that may have been moving at an even slower speed.) ...

Most tractors operating on public thoroughfares display a triangle indicating that they are slow moving vehicles. Other vehicles (the taxi in this case) traveling at speeds so slow that they could endanger other motorists are considered to be in violation of most vehicle traffic codes.
Regardless, all motorists (the motorcyclist in this case) are expected to have their vehicles under control at all times.
The investigating officer will probably issue violation citations to both the taxi and motorcycle drivers, as they seem to have both been at fault.
The only thing that might keep the taxi driver from getting a violation citation for driving too slow would be if he had been signaling for a turn.
I only have two questions ... If there was NO ONCOMING TRAFFIC, why didn't the driver of the motorcycle pass both the tractor and the taxi? And, did both the taxi driver and the motorcycle driver pass their sobriety tests?
Thanks,
Tim, for allowing me to have some fun with your Playmobil Police Department! ...

All the best,
Richard