In July 2011 a group of 8 reported: “We stayed at the ETAP in Cergy coming in on the D92 from the D98. It was totally fine there, and there was a restaurant 100 yards away which was also good. For a place to get your head down and a decent meal, this part of Cergy was fine, and all eight of us thought that.” In August 2011 another said: “ We stopped at the F1 very budget motel on the outskirts, only 3 mins from the fields. For 33 Euros per room, with shared showers and toilets, it was OK for crashing out. Eating isn't convenient, but there is a Campanile restaurant and Coutepaille grill a mile away.” (This group had motorised support)
From Marines: D28 to Ableiges, then D92 (Rue Louis Pasteur) to Montgeroult, effectively tracking the railway. About 300M after Boissy L'Aillerie station, right onto D22. This [50M climb] takes you over the A15 autoroute and into the business park of Cergy. Left at 2nd roundabout (Bd de la Paix) from where, if I recall, the hotels are signposted: left again at 2nd roundabout, then right into the road on which the Etap sits. We hit Cergy in the busy evening rush-hour and in drizzle, but there are bike lanes throughout and it's safe enough, as long as you take into account the unique Gallic attitude towards roundabout discipline: as ever, assertiveness pays.
To rejoin your route the next morning, the initial challenge was to find the Bd de l'Oise (wise after the event, I now realise this should have been simple, but we're in roundabout land, with no locals around...) then we followed this (busy, but safe) main road over the Oise to Eragny, turn right onto D984, then 3km south to the Seine, turning right onto the Quai to find the pedestrian and bike bridge across the river. This was pleasant suburban riding, mildly downhill, paying us back for the effort of the final climb the evening before. South of the Seine, there followed a hairy kilometre or so (lots of aggregate trucks etc) on D30 until we got into Archeres, then Route des Loges, which takes you into the Foret de St Germain. Then it was a case of choosing the right paths having turned right at the Maison d'Education de la Legion d'Honneur, eventually to rejoin your printed route at the Etoile de Beaumont on Sheet 8.
Probably about 12 miles from Marines to Cergy, then the same again (apart from getting lost a bit) to rejoin the next day, so this detour doesn't really alter the overall mileage of your route.