Here is the full list of all 18 Tiny Traffic achievements.
Crash 3 or more cars simultaneously
Solve all puzzles in Chapter 4.
Get to the final level.
Crash two cars into each other, inside a tunnel.
Use a tunnel for the first time.
Finish 100 cars.
Solve all puzzles in Chapter 3.
Unlock Chapter 4.
Solve all puzzles in Chapter 2.
Unlock Chapter 3.
Use the reset button 42 times.
Crash into a gate.
Crash 15 cars.
Solve all puzzles in Chapter 1.
Unlock Chapter 2.
Crash into a barrier.
Crash your first car.
Finish your first car.