Acceleration is not about how fast you are going — it is about how fast that is changing. The $\Delta$ in $a = \dfrac{\Delta v}{t}$ (read aloud: a equals delta-v over t) means final minus initial, and that subtraction carries a sign: pull away from a light and $\Delta v$ is positive; brake to a stop and $\Delta v$ is negative, so the acceleration is negative too. The sign is not decoration. It is the direction of the push.