David Fenton asked Chris Waigl and me: “Is there a mixed usage of “en route”, “in route” and “on route” that is common, or am I hearing a connection between three independent phrases that doesn’t really exist?” It turns out that the “on” and “in” variants of the French “en” are very frequent indeed; raw Google web hits for “— route to” on 22 September 2005:
en route to: 5,930,000
on route to: 265,000
in route to: 192,000
A quick glance at a sampling of the “on” and “in” examples should convince anyone that these expressions are synonymous. The version with “in” translates the French literally. The version with “on” is an especially good translation of French “en”, since it occurs
with the English noun “route” in expressions like “on the/our route to Vancouver” (where French “en” is just unacceptable, in writing or speech). Note the nice juxtaposition of “on route” with “on its route” in the first cite above, from the MSN Encarta dictionary’s entry for the noun “stop”. In any case, both “on” and “in” represent attempts to Anglicize, and make sense of, the French expression, and both are phonologically very close to French “en”.
This reshaping seems to have gotten by under almost everybody’s radar (neither of the Anglicized variants is in the current OED, and even Garner doesn’t complain about them), though at least two of the standard sources mention it: Brians instructs us not to Anglicize “en” in “en route” as “in”, and MWDEU has an unusually stern entry labeling “on route” as an “embarrassing error” and cautioning: “Authors and proofreaders beware.” (Brians doesn’t mention the “on” variant, and MWDEU doesn’t mention the “in” variant.) Despite them, I think that these variants are fast edging into the mainstream.
[CW, 2005/09/27: Added “on mass(e)”, as suggested by Sandi in the comment section. The partially Anglicised form “on masse” might simply be a misspelling of the French preposition _en_, though.]