I was hoping it was actually a statement, rather than a request: Cyclists slow [because they're polite]. But 'slow' is a transitive verb, which ruins the happy theory.
Stencilling 'Cyclists slow down' would be even more of a statement, while 'Cyclists: slow down' uses up more white paint and tarmac. Otherwise, as wingpig noted, a simple 'SLOW' might have sufficed.
Does the Road Traffic Act differentiate between 'Slow' and 'Dead slow'? The latter is often accompanied by signs reading '5mph'. And as kaputnik says, we all have different takes on slowness.