No matter how beautiful the path where people walk, no matter how complete and cooling the canopy of street trees, and no matter how handsome the street furnishing, perceived danger trumps them all. Automobile traffic speed is the universal danger. In the seven Walk Appeal conditions just below, an increase of just 20% in vehicular speed drops the Walk Appeal of the thoroughfare to the next lower condition. A Great Street should have traffic at no more than 15 miles per hour, where the risk of serious injury or death from being hit by a car is close to zero. But if traffic speed increases just 20% to 18 miles per hour, the Walk Appeal of a Great Street drops to that of a W-5 Main Street. Following this all the way to the bottom, a thoroughfare indistinguishable from a Great Street in any physical characteristic that allowed vehicular speeds of 45 miles per hour or more would have the worst Walk Appeal rating of W-0, which is Unwalkable. No matter the beauty, people don't want to be right beside sudden death.
There's one important caveat: the speed that matters is in the travel lane closest to the walking path. A Parisian boulevard moved cars at lethal speeds in the middle travel lanes the last time I was there in 2016, but cars on the slip roads at the edge with parking on both sides moved not much faster than walking speed, so sidewalk cafes flourished with high-speed traffic only four lanes and one median away.
Substantial physical barriers between automobile travel lanes and sidewalks create comfort commensurate with the mass of the barrier. Mature street trees are great at this, but there's a catch: transportation engineers consider trees to be "fixed hazardous objects," so they do everything they can to ensure that any street tree planted will not grow large enough to imperil the driver. No matter the impact to anyone else. We experienced this firsthand in the Flamingo Park Neighborhood's struggle against the Florida DOT. The DOT conceded to a median, but allowed only one street tree per median right in the middle of the block and mandated highly-packed soil around the tree so it would never grow much thicker than your thumb. For this and many other auto-domination decisions in the Alton Road rebuild, carnage has ensued.