In Sonic and the Sega All-Stars Racing (iPad port. Oh, well, probably technically iOs port), you cannot "accelerate" - the game does that for you automatically.
At the same time, when I time-race, I can see the "ghost" version of myself takes off at the beginning of the race. I did a bit of reading, and found that in the console versions of the game you can "accelerate" at the start if you hold the accelerate button at the start of the race.
Is there some alternative to do this on the iPad?