French and British police have concluded that driver Henri Paul, acting security chief at the Ritz Hotel in Paris, was well over the legal alcohol limit.
Paul died in the crash, as did Diana's boyfriend Dodi Fayed.
John Searle, who was hired by the Ritz Hotel, said drinking was not necessarily relevant.
Searle testified that the amount allegedly consumer by Paul would have increased his risk of accident 10-fold.
That meant, he said, that Paul's risk of having an accident while driving 2.5 miles (4 kilometers) was the same as that of a sober driver traveling 25 miles (40 kilometers).
"There is a risk, but it's a small risk, it still remains a one in many thousands chance of having an accident," Searle said.
"The accident rate is increased ... by a very significant amount, but that does not mean that it must necessarily have a bearing upon what has happened and why," Searle said.
"There are other factors also that counterbalance that one."
Fayed's father, Mohamed al Fayed, holds the lease on the Ritz.