Slipstreaming Passing Vehicles When I'm alone and plodding along the shoulder of a state highway, I like to stand up to catch a wind gust as large vehicles "fly" by me. The amount of "fun" I get from this is exponentially proportional to the size of the passing vehicle.  Unfortunately, I have a few concerns regarding my mischievous behavior. 

Since my average speed is about 15 MPH [1], I'm typically only in the slipstream of the said vehicles for a short period of time. Does standing up for this short period of time actually give me a substantial increase in speed?  

Also, I often wear highly recognizable clothing and/or gear.  There may be a few non-cyclist motorists that recognize me.  I'm concerned that my act of standing up right before I'm getting passed could be perceived as "aggressive" behavior.  If I continue to stand up to maximize my slipstream from passing motorists, do you think I will encourage some motorists to dislike me more than they already probably do?

[1] - My maximum speed is 40 MPH but that's downhill.  Also, I can typically get up to 20 MPH on a flat way when there is a strong tailwind.
This seems perfectly fine.

Cyclists often stand up for various reasons, and there's no reason to think that it would startle drivers. They probably have no idea why you're standing up anyway. Some people may eventually realize that you stand up every time they pass you, but I doubt they'll be anything but mildly amused.

Actual drafting (riding behind a vehicle) can and does stress drivers out on top of being rather dangerous for the cyclist.

As to the speed effect, standing up will make you go faster if the air is moving faster than you are. Effectively putting this into practice has a lot of nuances though, and really should be a separate post.