Why in the Book of Exodus did God let the people travel for exactly 40 years? What is the symbolic idea behind this number?

Also, what might be His intention behind the travel?
The field of Biblical numerology can be a pretty mirky pond. There are some clear trends (three for completion, seven for perfection, a thousand for a really big number, etc.) and a lot of speculation and conclusions that may or may not be warranted.

In this case however, the answer is pretty much spelled out in another verse. The 40 years of wandering was an exact calculation, one year for one day, based on how long the spies spent looking around the land before they were originally supposed to enter.


  Numbers 14:34 (ESV)
  34&nbsp;  According to the number of the days in which you spied out the land,  forty days, a year for each day, you shall bear your iniquity forty years, and you shall know my displeasure.’


The logical next question is, "Why did the spies spend 40 days on their quest"? To that we are not given any definite clues. It does seem like a reasonable amount of time to get around to a bunch of different places on foot and get an idea of the lay of the land without making your movements too obvious and getting caught.

There are actually quite a few references to 40 in the Bible. The flood. The spies. The wandering in the desert. The reigns of many of the Judges. Moses life has three periods of 40 years each. The temptation of Jesus. The surmised time between the resurrection and the ascension that Jesus spent back on earth. However the connections between these references is faint at best, and I think it is safe to say, since we are not told explicitly that there is any meaning to this number, that it is of no particular consequence that the spies took 40 days to get their spyin' done.