We have all been there - stuck in traffic, nose to tail. Nothing to do except hum along to a banal radio tune or fix the make-up (though what the guys do, I cannot say!).
If your business mostly operates in your local area - the places you drive to most days - then your rear is definitely going to be your most cost effective form of advertising without exception. It may be Friday afternoon, but we are definitely talking the rear of your vehicle.
Leaflets, adverts on radio or paper - all have a shelf life. But the life of your vehicle will be longer than either of those. Dependent on the design, rear car decals (an Americanism for you) raise your profile in the area, remind people you are there, direct traffic of the internet kind and, most importantly - Get You Noticed.
So for just £35 for a single colour, or £40 for two colours, rear car decals should definitely be on your marketing hit list.