Louvain-la-Neuve was designed as a pedestrian city, and so it is. You won't have to walk more than 20 min in order to get to any place in the city. Moreover, there are lots of covered paths in the center in case of rain.
Motorized traffic in the center is entirely underground, so it's pretty enjoyable for the pedestrian. The city is also very accessible for wheelchair, bikes, rollerblades...
Be careful, nearly everybody gets lost the first time they get around the town, as everything looks pretty much the same from an outsider point of view, and Louvain-la-Neuve's layout is not very logical to new visitors.
You can find a map of Louvain-la-Neuve here click on télécharger le plan to download.