How's this for an attention- catching blinkie? I spotted this on the website of a traffic-counter vendor, Gulfcoast Solutions.
They're about 0.6m wide and 0.3m high, and run off 2 AA cells ("alkaline recommended" - probably you'll be changing batteries daily). They certainly would give a clear message to pass you on the left! (But what if you ARE already on the left - will you get impaired drivers driving up on the curb?). Anyone want to get in on an order to try them out? They're cheap, only $20US at the present.