The employees have no control over the speed. SFMM and no other park for that matter would have trevor the teen and derek the surfing dude mess with computer stuff, plus all of that equiptment isnt at the area where they work anyways, its located in a different area of the ride.
Just like any other ride how high it goes is effected by temperature, car load (weight), and wind speed (headwind and tailwind). it is hard enough trying to program the ride to launch period let alone keep changing it.
You can see the effects of the above condiditions clearly at Nascar Cafe as Speed the Ride will sometimes hit the high brake and sometimes end up a long way away from it.
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"ok everyone go ahead and pull down on your shoulder restraint so you feel nice and stuck!"