Saturday, April 28, 2012

Stop Light Rail

Linear momentum is mass times velocity:
\mathbf{p} \equiv m\mathbf{v}.

The momentum of the oncoming train bearing down on you and your stroller, whose wheel is caught between the rail and the pavement, is the mass (weight times gravitational acceleration) multiplied by the velocity:
\mathbf{\bar{v}} = \frac{\Delta \mathbf{d}}{\Delta t}.
You try pulling the stroller from different directions and nothing seems to work, while wondering if the gravitational acceleration might be increased by the declining slope.
acceleration components

The gravitational acceleration doesn't matter as much as the friction (~0) of steel wheels on steel rail or operational inattention (∞), as the stroller seems to want to collapse instead of decoupling from the rail.

rail wheel and bus tireIf only that were a bus bearing down on you and the stroller. A bus is much lighter than a 51+ ton light rail train (25 or 26 tons per light rail train car). Bus tires are wider, made of rubber, and run on pavement (Friction City). Bus operators are actual drivers; they can brake for pedestrians, steer the bus, and are used to reacting to their environment.

This isn't theoretical; light rail kills. The trains are unstoppable.

Yesterday's post included the story of a mother pushing a stroller who was struck and killed by a Metrolink train last October in Riverside, California. She had been pushing her two year old toddler when the stroller wheel got stuck in the track. The mother was able to push the stroller out of harms way, but she lost her own life. The train couldn't stop; they never can.

What does it take to stop light rail?

The stopping distances are compared at Stop Light Rail, but it's metric and somewhat of an over-simplification. Gravitational acceleration is not a constant. Trucks, buses, and cars have differing stopping distances. But the concept and the ratios are there.

Bombardier Flexify (Hiawatha Line - Blue Line) v. Siemens S70 (Central Corridor - Green Line)

Light rail manufacturer Siemens says the Siemens S70 (25 tons per car) has an emergency stopping rate of 4.9 mphps (in good weather, on a level grade). The key word is emergency. The standard stopping rate of the Siemens S70 is 3 mphps. The Hiawatha Line's Bombardier Flexify (26 tons per car) standard stopping distance is 1.2 mphps.

Miles per hour per second (mphps)

The formula is SD = v² / (7200 x BR)
     where SD is stopping distance in miles
     v = speed in mph
     BR = average braking (deceleration) rate in mph/sec

Digging through reports instead of using the math, we find a National Technical Information Service (NTIS) order #PB-254738 titled Automatic Train Control in Rail Rapid Transit - May 1976 - appendix A page 179, says:  While [people] can act to prevent some accidents, [they] cannot prevent all of them, partly because [they] simply cannot stop the train in time. If one were to assure an instantaneous response and brake application along with a rather high braking rate of 3 mphps, it can be calculated that the minimum stopping distance from 60 mph for a typical train is 880 feet, and 220 feet at 30 mph. Clearly, there are many situations in which the potential hazard is either not visible at this distance or is created within the stopping distance of the train, (Suicide attempts are the classic example here.)

Car Stopping Distances

To put braking distances in perspective, Consumer Reports compared the dry/wet stopping distances in feet from 60 mph of six common cars and pickup trucks (model year 2012) in their September 2011 magazine. They said the Ford Focus hatchback stopped at 137/146 feet, the Honda Civic at 143/158, and the Ford F-150 pickup at 142/155.

Car and Driver magazine says the 2012 Ford Focus has a 173 foot stopping distance from 70 mph, and the Honda Civic Hybrid has a 196 foot stopping distance from 70 mph.

Motor Trend magazine says the 2012 Camry Toyota LE drops from 60 mph to zero in 120 feet on dry pavement, the 2012 Hyundai Sonata GLS stops at 128 feet, the 2012 Volkswagen Passat SE at 130 feet, and the 2012 Chevrolet Corvette stops at 94 feet.

Again, the stopping distance of a 60 mph train that can stop at 3 mphps is 880 feet. From 30 mph, it's 220 feet. According to Eddie Wren of Drive and Stay Alive, icy conditions can cause braking distances to be ten times longer than on dry pavement (or rails).

Light Rail Braking Systems

An engineering report from the University of Maryland says light rail trains have three stopping mechanisms: standard braking, emergency braking, and track braking (rarely used). In accident investigations, a key unasked question is, "Which braking systems were applied?"

 Examining the crashes listed in the previous post, there are some common threads:
       light rail trains can't stop
       light rail operators don't pay attention (not that they could do anything anyway)
       people -- pedestrians & drivers -- do stupid things (and don't have an overview of what's going on)
       light rail trains do not belong on roads

The news stories show many train accidents in Minneapolis, Los Angeles, Denver, Portland, Houston, and other light rail cities. Trains that run underground (subway), elevated, or (here's a thought) through rail yards (instead of front yards) have fewer accidents.

According to the Independence Institute: accidents point out the key flaw in rail transit: It is simply not safe to put vehicles weighing hundreds of thousands of pounds in the same streets as pedestrians that weigh 100 to 200 pounds and vehicles that typically weigh a few thousand pounds. Heavy rail (subways and elevateds) avoid this flaw by being completely separated from autos and pedestrians, but are still vulnerable to suicides. Light rail, which often operates in the same streets as autos, and commuter trains, which often cross streets, simply are not safe. Aside from being lighter than railcars (and thus less likely to do harm when they hit you), buses have the advantage that they can stop quicker. Rubber on pavement has more friction than steel wheel on steel rail, and the typical bus has many more square inches of wheel on pavement than a railcar. No matter how good the brakes on the railcar, it is physically impossible for it to stop as fast as a bus, for if the brakes are too good the wheels will just slide. This is why light rail kills, on average, about three times as many people for every billion passenger miles it carries as buses. Commuter rail kills about twice as many people as buses. Only heavy rail is safer than buses, and then only if you don’t count suicides. Autos on city
streets are a somewhat less dangerous than commuter rail, while autos on urban freeways are
somewhat less dangerous than buses. Safe transportation thus means more freeways and buses, not more rail transit.

Stopping Light Rail
             Policy Analysis - The Case Against Rail Transit (Cato Institute)
             No Light Rail (Vancouver)
             No Light Rail on Byron Avenue (Ottawa)
             No Tax for Tracks (Florida)
             No Tolls, No Light Rail (Vancouver)
             No Train (Wisconsin)
             RIP Gateway Corridor (Minnesota)
             Stop Light Rail (Paradise Waters, Queensland)
             Stop light rail in its gold-plated tracks (Seattle)
             Stop Light Rail, Now (Virginia Beach)
             Stop the Light Rail Obsession (Minneapolis)
             Wham-Bam-Tram (Houston)

Shouldn't it be safe to stroller down the street?

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