TODA=ASDA has some significance to the civil engineer designing a runway .. I will refer this to Overrun to get his view ... but is of little interest to the pilot or ops engineer
The detail of the discussion in this topic goes beyond the ambit of the airport civil engineer. Their planning of runway length is necessarily more prosaic. The myriad combinations of aircraft type, engine rating, load, and fuel are almost limitless. The aircraft type is often dependent on a particular airline, which over time changes equipment, routes and could even close down.
Since the structural life of the runway is typically 20 years, and the geometric life (its alignment) could be 50 years, it would therefore be foolhardy to precisely tailor the runway to an operation. What is done is a more generic approach to the design of the runway by the airport civil engineer. This leaves the task of fitting an aircraft/route to the runway (and its obstacles) to the aircraft performance engineer. And the task of day-to-day fine-tuning of load, wind, temperature, runway surface conditions, aircraft empty weight, and engine rating to the pilot.
There are several airport civil engineer approaches to designing runway length. The simplest is the FAA airport design program V3.2 which gives some generalised runway lengths:
Large Airplanes of 60,000 lbs or more Stage length of 1000mi 5950 feet
Stage length of 2000mi 7600 feet
Stage length of 3000mi 8950 feet
Stage length of 6000mi 11,200 feet
FAA also has performance charts for the (older) aircraft. ICAO has their reference length approach. Because of the high cost of a runway, it is more common to use the manufacturer's Airplane Characteristics for Airport Planning manuals for a detailed design. With these, the design aircraft is forecast, the takeoff weight is determined (usually MTOW), and the aerodrome elevation and design temperature are used to find takeoff runway length required. Nil wind is usually assumed. There is a mechanism for adjusting for the runway slope. The runway length for landing is similarly checked. The design length is the longer of the two – almost always the takeoff length. This gives the basic runway length so that detailed civil engineering design can proceed. The airline is usually asked to do a performance analysis to refine the basic length, and they would incorporate their operational limitations, runway alignment length (i.e. line-up distance), and obstacles into the performance calculations. Out of this study would come any refinement of clearway and/or stopway.
As kenparry said, the balanced field length concept is a runway where ASDA (EMDA) = TODA. To me, as PJ Swatton says, it is the equality of the stopway length and the clearway length which is now referred to as 'balanced'. If the stopway does not equal the clearway, the field lengths are considered unbalanced. I looked at the declared distances for a dozen or so Australian airports, and they are generally unbalanced with the ASDA being < TODA. Very occasionally (Pearce 05) the ASDA is > TODA. However the frequency of "balanced" runways would possibly vary by country.
I'm not seeing any trend for providing stopways and clearways on new runways. The cost of the runway pavement, relative to the cost of getting the runway strip filled and levelled and the cost of the aerodrome overall, isn't excessive; an extra hundred metres or two of pavement is within most budgets. The incremental difference between building stopway pavement and building runway pavement isn't much. This thread has made me wonder whether long clearways have become redundant. The RESA rules, where RESA has to be provided and starts from the end of the TODA, means one now has to build some sort of pavement structure beyond the TODA anyway, plus a pavement structure to connect the runway and the RESA (i.e. along the clearway). Of course, where the runway IS space limited, then "squeezing a quart into a pint pot" may well see the necessity of providing either a clearway or stopway or both.
I have a feeling that it is now economically attractive to make the clearway shortish, so as to limit to overall length of facility provided. The whole thing can get rather long these days, you'll be pleased to read. I am looking at a 747 runway at the moment (medium haul), and we have approximately 3000m of runway (TORA). Add 60m for the runway strip at both ends. Add 240m of RESA at each end. That is a total of 3600m of length needed within the aerodrome boundary, of which the airline can only "use" 3060m for TODA.
So coming all the way round to JT's question – for the airport civil engineer, TORA, TODA, LDA and RESA are the direct design concerns; ASDA and achieving a balanced field length is less so. Having said that, this thread has made me think long and hard about what distances are declared and built. The debate over RESAs is hotting up amongst airport engineers as the deadlines loom closer for retro-fitting them, and this thread adds to our discussion.