My take on it. I would suggest a 6' x 18' banner. 150' is a pretty good distance.
I would suggest to make sure your banner is the largest or at least as large as the next biggest one. Try to get a slot selection from the event. I would go with an 8' x 20' banner. I have done plenty of 8' x 20' billboards and they can be seen from 200-300' away.
The above is a very good layout for a billboard. Chances are most billboards are viewed for a quick impression nothing more. But a design like that will get you a few extra impressions.
The only thing I would change would be the size of the Company Name. I would make it a little bit bigger and space out the space between each character a bit more...
The phone number could use a bolder font and changed to white. But chances are not many viewers will read down that far anyways.
In the end that design by SignManiac will get you noticed! I have designed and printed a couple hundred billboards. That is a good design for the most part, good enough to get your name out there.
In the end billboards are primarily for branding campaigns. Although they can work for smaller companies looking to launch a new product or sale.
If you are advertising with the at a track. You need to find a way to bundle up your marketing strategies. Don't just take a billboard and hope it gets their attention. Target your potential clients with multiple attacks...lol. Create some Roll Fold brochures or flyer and work out a deal to have someone hand them out to fans and/or drivers in the drivers meetings.
GL with your Ad.