Originally Posted by coffeecake

Hi. Sign shop employee here. I'm going to sidestep the debate about installing a sign in the first place for a moment and offer you some input.

If you want to get this made somewhere, you have a couple of options - cheapest option would be coroplast (corrugated plastic cardboard) with adhesive vinyl lettering. This should last at least four or five years. You can attach it with zip ties to a rear rack. If you want something more professional, sign shops should have a lightweight aluminum composite board - two layers of aluminum with dense foam in the middle - that's a bit more durable and easy to drill holes into; it's called di-bond aluminum in these parts. Another thing you could use is some heavier plastic - sign shops, especially those that screenprint parking tags and small signs should have sintra or styrene available, which come in many thicknesses and varying rigidity. Other shops may have a flatbed digital printer that you can print directly onto whatever surface you like.

It may be difficult to find a shop that will do this for you. Many sign companies don't bother with small, one-off orders like these, or they may charge you more than you'd be willing to pay. I'm not sure of your budget, but where I work, it's around $60 -$80 an hour for labour, plus materials. You could also ask for coroplast offcuts - many shops will give these away for free, if they're decent. You can get pre-cut vinyl lettering from Walmart as well. If you have had an election in your area recently, election signs are a free source of material.

Readability: my chart that I use at work indicates 3" letters will be visible at a maximum distance of 100', with maximum impact topping off at 50'. This is for red or black on white background. I don't have any charts with speed factored in; but keeping it short will help.

As to whether or not this will provoke motorists, I find that in many places in North America, simply being on a bike will provoke their ire. Personally, I would not do it, but I do not like confrontations, unless there is a possibility of cake afterwards.