Even if walkers, cyclists, and motorists have to travel in the same corridor despite differences in route preferences, a secondary question remains: Can they mix safely on a single public way? The answer hinges on their differing travel speeds.
Someone shuffling along a sidewalk is moving forward only about 8 khr/5 mph slower than the fastest jogger, but even that relatively narrow speed range requires passing lanes so everyone can progress at their preferred speeds. Meanwhile, roller-bladers can travel up to 16khr/10mph faster than walkers, and cyclists up to 32 khr/20mph faster. When these overtake speeds are expected, paths need to be at least 10 feet wide, and preferably wider so the fastest can maneuver safely past the slowest.
The side-to-side motion of expert roller-bladers requires even greater width for passing.
However, room for passing tends to make a multi-use path more dangerous because wide paths accommodate the highest overtake speeds. Narrow walkways create safety in one of two ways: walkers stay on the path slowing cyclists and roller-bladers to their pace, or walkers step off the path to effectively create segregated paths—the paved route for those on wheels and a treadway in the grass for walkers.
Are 10- and 20-mph overtake speeds important? A 2011 study by Hunter College professors suggest that they are. They found that bicyclists colliding with pedestrians send more than 500 New York City residents to hospitals each year. (http://cityroom.blogs.nytimes.com/2011/09/19/study-finds-higher-number-of-pedestrians-hurt-by-bikes/)
Cities have recognized the danger and are creating three separate circulation networks, one each for cars, cyclists, and pedestrians, much as Olmsted and Vaux created three networks in Central Park for carriages, horses, and walkers. In New York City, spokesman Seth Solomonow reacted to the Hunter College finding by saying the city would continue to work to “install bike lanes to separate vehicles from cyclists and cyclists from pedestrians. These make streets safer for everyone who uses them….”
Similarly, Central Park today isolates cyclists from runners and walkers even on roadways closed on summer weekends to motorists. It limits cyclists to 24 khr/15 mph at all times and places. The Conservancy’s logic is succinct: “Cyclists are not allowed on pedestrian paths, as this poses a danger to all pedestrians.”
Of course, in cities like Portland, Maine, which have few separate bike lanes, we cyclists tend to behave like cars because the short blocks and congestion keep car speeds at or below 30 khr/20 mph for the most part anyway.