OveRun,
In FAA parlance, the RSA encompasses the entire runway. The RSA width limits may be identified by the runway hold short signs as the must be located no closer to the runway than the RSA limit. There’s a takeoff RSA that extends beyond the departure end of the runway and a landing RSA that extends out before the approach end of the runway. However, if the runway is operationally bi-directional the takeoff RSA for both runway ends establishes the RSA length beyond each runway end. The illustration shown in the CARDAip07.pdf ICAO document linked earlier in this thread depicts this concept.
In the US, the RSA is intended to contain the aircraft in the event of runway excursion, laterally or off the ends. It must be graded and constructed (does not need to be pavement) to support the aircraft without undue risk of damage to the aircraft in the event of such an excursion. When the airport geography does not allow the airport operator to obtain the full RSA length beyond the runway end, then FAA allows the operator to declare a reduced ASDA and LDA to obtain the required length (or obtain as much as practical while not adversely impacting the runway’s usability).
In the US, ASDA may be less than TORA or TODA as necessary to obtain the required RSA length. My understanding is that in ICAO Annex 14, ASDA may not be less than TORA or TODA.
On another note, I failed to mention in my previous post that TODA may be reduced to less than the full, physical runway length (and less than TORA) if by doing so the airport operator can obtain a clear 40:1 OCS as defined in AC 150/5300-13, Airport Design (not TERPS…closely related, but different 40:1 surface). The allows the airport operator to establish a runway length that the FAA TERPS procedure designers will use in evaluating obstacles in the TERPS takeoff Initial Climb Area (ICA) and may result in the elimination of 40:1 TERPS OCS penetrations that would result in non-standard takeoff minimums and/or use of a takeoff climb gradient on a SID. There are some airports (I believe that one example may be found at Memphis, KMEM) where this technique is applied.
Rich