The Gaussian sampling strategy decides to keep a point based in part
on testing a pair of samples. This idea can be carried one step
further to obtain a *bridge test*, which uses three samples along
a line segment. If the samples are arranged as shown in Figure
5.29b, then the middle sample becomes a roadmap vertex.
This is based on the intuition that narrow corridors are thin in at
least one direction. The bridge test indicates that a point lies in a
thin corridor, which is often an important place to locate a vertex.

Steven M LaValle 2012-04-20