The Flash has evolved in comics over the years and is now capable of running at any speed, even faster than the speed of light. This allows him to time travel and be in several places at once.

 

In reality physicists believe nothing can travel faster than light. This is because the energy needed exponentially increases as you get closer to the speed of light (Not linearly like the Blue line, as newtonian physics predicts). This means that a small increase in speed towards the speed of light (the dotted line) takes a massive amount of energy (the Red line on the graph). You can never get enough energy to accelerate a mass to the speed of light.

 

The speed of light is 300,000,000 m/s