To clarify: The problem is the foot you put down slipping on the ground, rather than grip on the pedal?
I occasionally find that a problem on the Streetmachine, exacerbated by one or more of: MTB shoes rather than the grippier touring sandals or boots; worn-smooth tarmac; adverse camber; wet; the bonk; not holding the brakes on.
I sit forward in the seat when stopped, in order to better reach the ground (which moves my centre of gravity forward), and habitually use my longer leg. Not sure how applicable that is to the Speedmachine, but presumably is only an option with USS.