I have used his Davids's Rickshaw on the ground and attached to the back of a vehicle and it works great. When sitting in the rickshaw you are able to get OVER 180 degrees of movement with the Steadicam rig. It makes shooting profile from a moving vehicle simple, since the Steadicam rig is a couple of feet behind the vehicle it is mounted to (clear field of view). Great for shooting profile of a person running, a bike or a car then following it as it slows and makes lane changes behind and around you. I felt safe and secure, and did not notice any slack or wiggling. You can always add rachet straps and don't forget that the the driver is one of the most important pieces of the equation. I would always like to have a Grip in the back near me in case I need a pair of hands in an emergency.
Side Note: When I am shooting from a vehicle (except for the Shot Maker) I have found it neccessary to include the total weight of all the people and the gear when we are test driving the route looking for any issues that may come up (potholes, incline, etc.). You can notice in the photos that the pickup is sitting lower in the back since there is so much overhang weight. Lets hypothetically say that the photo was a rehersal run, but when picture is up, the 1st AC, and a Grip will usually be in the bed near me. The Director, DP or Scripty may want to sit in the crew cab with a monitor, or worse and less safe, they want to sit in the bed of the truck, There is a very good chance that the vehicle could unexpectedly bottom out causing pain and damage.
I really like that David's rig gives more options to the production. It works great as a standard rickshaw on the ground, and then can be quickly mounted to a vehicle for shots that need a little more speed.
Ari Gertler