Not to interrupt you Lawyer types and legal arguments, but...
The roads are the roads are the roads.
They may be covered in debris, including (but not limited to) sand, mud, gravel, leaves, dead critters, blood, radiator fluid, motor oil... etc.
Not to mention potholes, gutters, streetcar tracks, curbs...
The 'real world' roads don't resemble manicured racetracks - not at all.
Now, GTAMers know this stuff already.
The reason I mention it is because, riding in the rain is dangerous; especially if the pavement is not clean! As the OP will testify.
There can be no reasonable expectation of the road not having grass on it (just like everything else). It is there.
It might be there tomorrow; it might be there next month...
And it is after all only a 50 km/h zone, anyway.
Point is, this is what happens! Trying to blame or sue someone isn't always going to cut it.
Maybe the local (Jamestown) kids threw grass on the road - who knows?
But you can't be trying to sue somebody every time you have a wipeout, thinking you have been promised perfect, spotless roads...
Because you have not.