Back in 2006, however, when a couple of teams first postulated their use I was asked to look into it. The following is my response;
Following the request to investigate any research into the use of mountain bikes for SAR I undertook both internet searches and posted requests for information on SAR forums and discussion groups. Although several suggested contacts were given I found no evidence of any serious research into their use.
Robert Koester, who has carried out sweep width experiments in the US, responded stating;
“I’m not aware of any sweep width studies done yet for mountain bikes.
…
I would expect the results to be similar to what we found with mounted
searchers. Ground searchers had a larger (better detection index) sweep
width off-trail in challenging terrain than the mounted searchers who had to
contend with directing the horses. On the open road the mounted searchers
had a larger sweep width due to a height advantage and little to no need to
concentrate on the “next step”.”
In the absence of empirical data, therefore, I tried to do the mathematics of using a mountain bike team for searching a route and path compared to a three searcher foot team – comparing different variations of speed and sweep width to see whether in the first instance it was appropriate to use mountain bikes for search and whether their use would change with the search environment.
Before I present the conclusions however, I must state that although I am confident of the results it would be best to conduct proper sweep width experiments to assess both the appropriate speed for bike search teams and sweep width estimates.
The aim of all searches is to maximise the Probability of Success (POS) of the search. POS is a product of the Probability of Area (POA) and the Probability of Detection (POD). Therefore, comparing two search resources searching the same area it is the POD that is the relevant factor.
The POD of a search resource in an area is a factor of the size of the area against the search effort put into it. This is described by the Coverage of the search resource, which is the distance travelled by the search resource times by the sweep width.
The example of a three searcher ALSAR foot team covering a 1km route and path is as follows:
Area to be searched: 2m wide track plus 5 m either side gives a 12m wide track by 1000m length equals 12 000m2 total length.
Distance travelled by searchers: Assuming a straight line and no purposeful wandering, 1000m by each searcher, 3000m in total.
Sweep Width: Taking a rough average sweep width figure of 54m (the 2004 experiment results ranged from 16m to 142m)
Therefore the search team’s “Area effectively swept” was 3000 x 54 which equals 162 000m2. This gives a Coverage of 13.5
Using Koopman’s exponential detection function we can translate this to a POD of 99.99%
Doing the mathematics of the bike team is slightly more complicated. According to the advice given the first bike in the team is purely there for navigation – to give warning of hazards etc. They, therefore, have a vastly diminished sweep width figure.
The two further bike team members can concentrate more on search but it must be recognised that they still will not have the same detection index that foot searchers will because of the need to concentrate on cycling. For the first calculation I will assume that they have half the sweep width figure i.e. 27m.
Area to be searched: Remains the same at 12 000m2
Area effectively swept: For the first bike 1000m x 2m (it is assumed they would notice a body in the middle of the track!). The two remaining bikes cover 2000m x 27m. This totals 56 000m2.
This gives a Coverage, therefore, of 4.6.
Translating into a POD of 98.9%.
The advantage of the bike team, of course, is that rather than taking approx. 30 minutes to cover the track as an ALSAR foot team would at a searching speed of 2km/h, they would take approx. 12 minutes at 5km/h.
This means that their probable success rate (PSR – an important calculation using the Charnes-Cooper algorithm) is greater than that of a foot search team and, therefore on the basis of these rough calculations, should be used in preference to the foot team if both are available.
There are a number of caveats to that, however.
Without doing the mathematics for each case it can be proved that the bike search team is of far greater benefit used in areas with a higher sweep width figure (i.e. less dense woodland etc.) and where they can travel faster (because of better tracks etc. i.e. less need to concentrate on riding.)
As the search environment becomes thicker, or the team have to slow down more and concentrate upon riding more their effectiveness diminishes. At very low sweep width figures the bike team would have to repeat their search area or a new resource allocated for the search manager to have a reasonable confidence that the misper was not in the area.
Any advice to search coordinators, therefore, should recommend the bike team’s use for easy to ride tracks in less dense woodland or open areas. A foot team would be more suitable for a harder to ride, denser search environment. (Obvious I know, but provable and worth stating)
What needs to be done, however, is to carry out field trials at the very least (preferably sweep width experiments although these require greater manpower than might be available). These would need to ascertain the “best” speed (or at least what the average speed for the bike search team is) and some sort of detection index (again at the very least the AMDR for the bike search team). These would allow much more accurate mathematic investigation and proof.
The trials that Jennie writes about in her comment were part of Wilsar’s response to my last paragraph that at the very least field trials needed to be run to satisfy us that it was a useful efficient search resource.
