As Americans, we don't customarily put these kind of stickers on our cars. Sure we've all seen the "Jesus is my co-pilot" bumper stickers, but these are totally different from the "safety in traffic" sticker.
Across the top of the sticker are four Japanese characters (kanji): 交通安全. The first two characters, 交通 (koutsu) mean "traffic," and the last two characters 安全 (anzen) mean safety. Beneath the "safety in traffic" characters is the name of the shrine whose god is looking out for you and your vehicle, which in this case is my favorite-and coolest-shrine in all Japan: Washinomiya Shrine, 鷲宮神社; the shrine of the small town I lived in for two years. 僕の鷲宮の友達宮内Shinya君が「交通安全」のステッカーを僕に送りました。本当にありがとうございました内友! Underneath all the writing is a drawing of the shrine gate and the eagle (FYI: "washi," as in "Washi-miya" means eagle 鷲).
As I've written about in earlier posts, most if not all Japanese people are Shintoist--at least culturally. Shintoism holds that all objects contain a god, kami 神. Things like trees and stones, rivers and mountains all have gods in them. That's peaceful to think about. But man made things also contain gods, like coffee mugs, pencils, and even, you guessed it, vehicles. In fact, it's not uncommon in Japan to see a new car and its proud owners being blessed by Shinto priests in the middle of the shrine grounds. Nothing like seeing a Mercedes getting blessed. (Click here to watch a Shinto car blessing video on YouTube.)
When cars are blessed (if that's the right word for it), the priests bless the car's spirit and ask that the car be a good and safe car for its new owners. After the blessing, many Japanese people will buy a "safety in traffic" sticker from the shrine and put it on the back window. Some shrines also sell "safety in traffic" stickers for bicycles, which are both smaller and cheaper. The going rate for a car sticker is 1,000 yen ($10) and 500-800 yen ($5-8) for a bicycle.
Even though my motorcycle wasn't blessed at Washinomiya Shrine, I believe it still has a spirit or god in it--after all it is a Yamaha--and it's nice to know he's being taken care of. My "safety in traffic" sticker helps keep my motorcycle safe and functioning properly, and that keeps me as safe I can be with my helmet on.