If you think that standard car signaling is not enough, then consider following project built by Robert. Its a car rear window LED sign controlled by Arduino Mega. This is a matrix of 48×10 red LEDs that are soldered sparsely so it covers whole rear window of the car and still allows to see through.
Such amount of LEDs are driven using a 4017 decade counter for rows and bunch of 595 shift register for columns. Microcontroller scans through columns at frequency which gives illusion of whole picture. Power supply of sign is attached to headlight power so it is only on when lights are turned on. This way you don’t have to worry about accidentally leaving sign on when parking. After build Robert noticed that LEDs are too dim to be visible in daylight. So the whole fun is seen at dark time.