You see inexperienced road cyclists in Japan riding in the gutter all the time rather than asserting their right to be on the road and taking their fare share of the lane. Its a dangerous practise for many reasons not the least of which is most gutters have drains at regular intervals so to avoid these the cyclist must swerve periodically. From a motorists point of view this is erratic and unpredictable behaviour which won't earn cyclists any respect on the roads.
Thanks Yokohama for giving us half a bicycle lane.
If you really want to enjoy Yokohama by bicycle I recommend you get in touch with James at Yokohama Rides and Rentals.
James can take you on a guided tour of the Yokohama and Kamakura areas and also has compact folding bicycles available for rent. As part of the Yokohama Rides and Rental service they can deliver bicycles to your hotel or meet you at the nearest convenient station.
For more information or to make a reservation please contact James at firstname.lastname@example.org or give him a call on 090-7849-6683.
Update: James visited the site of this new bicycle lane, made some observations and provided Tokyo By Bike with pictures confirming drains in the "lane" and other hazards. Read more in the article "Yokohama Bicycle Lane is a Real Obstacle Course".