Figuring the difference between the two is one-reason pilots spend so much time with charts and flight plans. Most pilots don't fly by instrumentation in poor weather, because of the rigorous training and regular updates that are needed. Anyone who has flown with a HUD will tell you "With a glass screen showing a highway in the sky and giving approach guidance based on real time conditions, a pea soup sky becomes clear as day."( ) .
Task saturation is a situation that any experienced pilot is familiar with. Situations like these are often the starting point of aircraft accidents. Task saturation is especially dangerous when it occurs at night or during bad weather. The pilot's inattention to flying can and often does manifest itself by a loss of situational awareness. "An analysis of 621 fatal accidents involving airliners around the world between 1980 and 1996 showed that 41% occurred due to lack of positional awareness." ( ) It is number's like these that started the Federal Aviation's or FAA's Safer Skies Initiative. The Safer Skies Initiative is a program in use to reduce fatal commercial aviation accidents 80% with half of those gains expected to come from HUD's or HUD technology. Nearly 40% of the accidents involved aircraft, which had not been fitted with ground proximity warning systems or HUD's. The most frequent cause of crashes were Controlled Flight Into Terrain or CFIT which is something that aircraft HUD's are very effective in preventing.
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Even with all of the current benefits of HUD's there is still room to expand the envelope. The University of Idaho in conjunction with the Air Force Office of Scientific Research is currently experimenting with the use of peripheral vision in heads up displays. The advantage of using peripheral vision to supplement normal HUD's is that the extra information doesn't require any extra attention-peripheral vision provides cues that impart information without the pilot having to think about it.