Monday, April 13, 2015

More on Walkableness on Busy Streets--Roundabouts/Traffic Calming vs Signs/Signals

Walkableness on Busy Streets Defined by Slowing Vehicle Speeds Along Streets and Intersections Through Traffic Calming and Roundabouts, the Way to Substantially Attain Walk Mode Safety at Busy Street Crossings

Residents and policy makers increasingly search for proper treatments on busy downtown and village center streets and intersections to in order to reach walkable status.  Generally streets of concern are designed for or feature 25 mph speed limits.

However you approach it, minimizing serious injuries and fatalities on busy streets for those on foot requires, first, reducing vehicle speeds at crossings and, second, cutting the spread between high and low speeds along streets overall.  This appears to apply to both intersections and mid-block situations.  Mid-block crossings are of particular concern becaust about a fifth of all walker deaths occur at mid-block. 

Traffic signals and stop signs may bring vehicles to a stop, but they also foster high speeds between intersections and crossings as well as high speeds at mid-block.  And in the case of vehicles at signals, racing the stale green or amber behaviors are routine occurrences.  Traffic signals and sign controlled intersections experience about the same level of pedestrian injury rates. 

Crossing treatments involving signs or signals in the absence of traffic calming treatment very likely retain unacceptably high rates of injuries.  The one treatment which drops serious injuries at an intersection up to 90% while also reducing cruising speeds up to two blocks distant is the roundabout.  The roundabout functions as an injury reducer at the intersection itself and traffic calms speeds few hundred feet in each direction.  It is the one treatment experience reveals where the more roundabouts installed the better the injury rates of all existing roundabouts.  And as a U.S. study of over 50 roundabout corridors through vehicle travel times vary little at roundabouts versus signs and signals mostly because lower between intersection travel speeds are balanced by reduced vehicle delay at the roundabout intersections.

A variety of pedestrian signals are being introduced in urban areas as mid-block treatment without any traffic calming at all, that is, without installing roundabouts nearby and no traffic calming (vehicle speed reduction) at the mid-block crossing location itself.  Traffic calming at the mid-block crossing can take the form of vertical calming like a raised crossing, or a horizontal treatment like a median diverter which forces a vehicle slightly off a straight path thereby lowering vehicle speeds at the crossing. 

As Mark T Johnson emphasized in a roundabout workshop in Burlington, VT in March 2015, the roundabout reduces the range of speeds—low and high—along a corridor.   At an intersection, the roundabout uniquely allows the average vehicle to move through the roundabout at 5-15 miles an hour versus a stop sign or stoplight constraining all or a sizeable portion of the traffic flow to essentially 0 mph.  Between intersections the roundabout also reduces speeds, traffic calms, so the difference between the lower “cruising speed” created by the roundabout outward a block or two and within roundabout speed is further reduced.  The difference between a stop sign speed, 0 mph, and cruising speed along at 25 mph street may easily be 25, that is 25 mph-0 mph.  A roundabout by contrast with basic vehicle speed at the roundabout of 8 mph may also reduce the cruising speed by 5 mph to 20 mph, so the speed differential drops to 12, about one half the speed differential found at signals or sign controlled intersections.  The reduced difference between high and low speeds on roadways does provide another measure determined empirically of how roundabouts cut serious injuries by about 90%.

In conclusion, it is fair to question the installation of traffic signals of any type without traffic calming of some kind if a basic purpose is to increase walk mode service and safety.  At mid-block crossings installing walk signals in the absence of traffic calming also leaves walk mode crash rates unacceptably high.  Ideally, mid-block crossings with the best walk mode safety involve both traffic calming elements at the crossing and roundabouts at adjacent intersections.