The Speed of Thought

The standard answer to most things about the Flash is his relationship to the Speed Force, an enigmatic superset of cosmic energy in the DC Universe, which allows the Flash to perform abilities beyond the laws of physics.

The Flash doesn't obey speed or motion laws.
The Speed Force makes him immune to the speed limits of the DC Universe. The Flash's powers, since the reboot of the new DC Universe, revolve around Barry Allen's return and his perfect connection to the Speed Force. It has been implied Barry is the Speed Force personified, making him the most powerful speedster on DC's Earth.

Flash Family, Jay Garrick, Wally West, Barry Allen, Impulse, XS, Max Mercury

Just as with his movement speed, he's able to control how fast he perceives time. So day to day, he spends his civilian time mostly perceiving time at the rate we all do. When fighting crime he'll vary it to keep up with how quickly he needs to be moving.

Issue #2 of the Flash (Oct 2011) shows Barry tapping into the Speed Force with just his brain and not his body. Through this, he's able to see what will happen in the near future to all the people in his surroundings. Click the above link to know more.

I covered part of this in my answer regarding "How does the Flash perceive time?" In the treatment I discuss that the nature of the Flash's powers are based around his ability to perceive, analyze and counter the attacks of his enemies.

Despite their silly names and often less than stellar outfits, the Flash's Rogues Gallery are not to be taken lightly, especially when they are working in concert. The bulk of the Flash's Rogues Gallery use weapons, techniques, or abilities that attempt to confound, distort, or alter the physical properties of the world around the Flash in an effort to prevent him from maximizing the use of his powers.

"Trapped in the Speed Force" is an expression taken from the comics where a speedster who is fast enough or in a particular level of synchronicity with the energy which gives speedsters their power, the Speed Force, could become ONE with said force and be unable to escape from the dimensional region where this power is said to emanate from.

The Short Answer: Because on television, he doesn't need to run at the speed of light for the television series to exist and tell compelling stories. He is only as fast as he needs to be. Right now, that is twice the speed of sound and it is perfect for storytelling with an ensemble cast. Why didn't they start him moving as fast as he does in the comics? It sure would be convenient for getting places. At the speed of light, Barry could circle the globe seven times in one second!

The Flash can alter the relationship of an object or person's movement in relationship to himself or other points of reference including the Earth itself, i.e. he can altering its movement in relationship to Earth's rotation, causing it to move away from him at a thousand miles an hour. The trapped object will plow through anything in its way. This would look like a needle skipping over a record.

The Flash's powers have evolved since the character's first appearance in 1940. While they were supposedly scientifically-based, little effort was made to explain any Flash's powers until the late sixties or early seventies re-creation of Barry Allen as the Silver Age Flash.

Jay Garrick who was once called the Golden Age or Earth-2 Flash, gained his powers from exposure to radioactive or "hard water." Despite this attempt at science, all of the Flash's powers may as well be considered magic since they depend primarily on the existence of the "Speed Force" as an underlying engine for their existence.

Despite how silly it might seem, unless otherwise specified, speedsters run like everyone else. But all speedsters are not created equally so each speedster, depending on the mechanics of their super-speed, might leave a different footprint or spread of footprints depending on how their powers function and how how they are based in their comic physics.

Because responsible physics makes for dull mythology
Legend sticks in the mind, long after the science has faded. Science solves problems. Spirituality soothes souls. Mythology is the fusion of story, aspiration and seeking to understand the universe through heroic endeavors which can be related to by the common man. Superheroes are modern myths.

Heroes are Judged by the Villains They Contend With
When you're the Fastest Man Alive, you need competition that can keep up. In the case of the Golden Age and Silver Age Flash, it seemed natural to give him an opposing enemy whose powers resembled his own, since the power of super-speed is both dangerous and visually stunning when depicted on paper. Normally, the Flash runs circles literally around his opponents, but what happens when that's reversed? Enter: Eobard Thawne - The Reverse Flash.

Disclaimer: Superhero battles within the same universes between different characters is generally a waste of time because the winner of said battle depends entirely on the plot. If one hero is needed to defeat the other, the writer will see a way to win and the hero who should win, won't. It's that simple.

The Tale of the Tape
With that said: We will take the current versions of the Sultan of Speed, the Master of Motion, without a doubt the Fastest Man Alive and pit him in a titanic struggle against, Earth's most powerful hero, dubbed the Man of Tomorrow, the Man of Steel or by his most distinctive sobriquet, Superman!