Good question.

Quote:

Marathon Timing Chip
Running is, in many ways, the simplest of sports. It doesn't truly require any equipment other than what you're born with -- heart, lungs, legs and feet. But technology made a big impact on race organizers when the marathon timing chip was introduced in the 1990s. Suddenly, they were able to track runners from the second they crossed the start line to when they reached the 5-kilometer point, to halfway through the event and the all-important finish line.
Timing chips are typically encased in a hard plastic ring and given to each runner in their race packet prior to the event. The morning of the race, marathoners tie the ring into their shoelaces. Meantime, antennas sheathed in wide, rubber mats are positioned along the course. When the marathoner steps on the rubber mat across the starting line, his individual chip is electronically recognized by the timing system, and his time begins. Likewise, any time he or she steps on one of the specialty mats along the course, a computer records the accumulated time to that point in the race.
Chip timing revolutionized marathon timing, but its drawbacks were recognized as well. While it offered accuracy and simplicity, chip timing was labeled by some as too expensive. Each timing chip ring had to be removed from the shoelaces of weary athletes at the finish line by race volunteers and returned to the given company that was timing the race.