Assuming you are landing on a horizontal surface, it's only the vertical component of the fall that you have to worry about. A fall from 2 meters accelerates you to about 22 km/h (27 km/h for 3 meters). Helmets are design to protect you from a 2-3 meter fall onto an anvil (depending on the standard). As long as you don't hit anything vertical (the curb, a telephone pole, a car, etc), a helmet will give you good head protection no matter how fast you are going (and will also give you some protection against abrasions).

Depending on how fast you are going, a helmet may even help for hitting things. Quite a few people don't exceed 25 km/h on their bikes, especially in city situations. Similarly, if you are aware of a dangerous situation, then slowing down may give you some protection. It's absolutely not going to help if you are bombing along at 40 km/h and get doored, but at that point, nothing really will.