Why is chip-time not considered as the official race-time from which finishing positions are calculated? (detailed answer)
First of all, it is important to provide some context behind the advent of chip-timing.
Chip-timing is a relatively new phenomenon and has only been introduced into road-running (and various other athletics events) in the last 10-15 years or so. The main purpose of chip-timing is to ensure that all competitors in mass participation events (e.g. large city marathons and at a much more local level, road-races such as the Streets of Galway) get a better approximation of the actual time they took to complete the event, given that not all competitors can physically be positioned exactly at the start line when the race begins.
Chip-timing has no official status however in determining "finishing positions in the field".
The reason it has no official status is that these events are first and foremost "races", where the principle of "first past the post" is used to decide the winner, the second-place finisher, and so on down through the field of entrants.
The winner is deemed to be the person who has the quickest race time (also referred to as gun, finish or gross time), which is the time elapsed from when the starter's gun (or other signal) is sounded to the time at which they cross the finish line. Furthermore, a race should not be confused with an individual time-trial (where the time recorded by each individual participant is most significant). In a race situation, how you compete and "race" against other competitors in the field is just as important as the actual time it takes you to complete the event.
Consider the (admittedly unlikely but still technically possible) scenario where a very fast runner starts from the back of the field and records the quickest chip (nett) time but does not have the quickest finish (gross) time and is also not the first person to cross the finish line. It would be unreasonable and unfair to consider him/her as the actual race winner in this situation, given that numerous other athletes may have actually finished the race before him/her.
On a much more technical level, in respect of our own road-race, the permit for the Streets of Galway is granted by Athletics Ireland (AI) as the governing body of the sport of athletics in Ireland. The AI in turn are affiliated to international athletics umbrella body, the IAAF (International Association of Athletics Federations). If a race is granted a permit by the AI, then it is the IAAF rules on race timing that apply by default.
The following extract from the IAAF rule book is relevant here (note - the formal name for a timing chip is "transponder"). The final paragraph makes the clear distinction between the merits of finish and chip times:
IAAF Rule 165.24 in Regard to Transponder Timing Systems
Page 114 and 115 of IAAF Rule Book
24. The use of Transponder Timing Systems approved by IAAF in events held under Rules 230 (races not held completely in the stadium), 240 and 250 is permitted provided that:
(a) None of the equipment used at the start, along the course or at the finish line constitutes a significant obstacle or barrier to the progress of an athlete.
(b) The weight of the transponder and its housing carried on the athletes' uniform, number bib or shoe is not significant.
(c) The System is started by the Starter's gun or approved starting apparatus.
(d) The System requires no action by an athlete during the competition,at the finish or at any stage in the result processing.
(e) The resolution is 1/10th of a second (i.e. it can separate athletes finishing 1/10th of a second apart). For all races, the time shall be read to 1/10th of a second and recorded to the whole second.
All read times not ending in zero shall be converted and recorded to the next longer whole second, e.g. for the Marathon, 2:09:44.3 shall be recorded as 2:09:45.
Note: The official time shall be the time elapsed between the firing of the starting gun and the athlete reaching the finish line. However, the time elapsed between an athlete crossing the start line and the finish line can be made known to him or her, but will not be considered an official time.