Both Monorail and light rail vehicles can typically reach
maximum system speeds of 55 MPH or more. However, who would want a 100,000 lb
LRT vehicle traveling down Lamar or Guadalupe at 55 MPH inches away from
automobiles, pedestrians, and bicyclists?
Light Rail as currently planned for Austin
would run in the center of the street. Light rail stations would be located in
the center of the street as well. Therefore to access the stations, pedestrians
and physically disabled persons must cross lanes of traffic. Light rail vehicles
can achieve a typical deceleration (braking) rate of 3.0 MPH/s or an emergency
deceleration rate of 4.2 MPH/s. An emergency stop from 55 MPH would take
(time = v / a):
55 / 4.2 = 13.1 s
and the vehicle would travel (distance = 1/2 * a * t**2):
0.5 * (4.2 mi/hr-s * 1/3600 hr/s) * (13.1 s)**2 = 0.10
mi = 528 ft.
This would clearly present a hazard to pedestrians,
bicyclists, and motorists. Therefore, LRT travel in non-exclusive right of way
is generally limited to 35 MPH or less.
The following study is a gold mine of information on the
safety issues and guidelines recommended to integrate light rail transit into
city streets:
TCRP Project
A-05, Integration of Light Rail Transit into City Streets
Because
Monorail typically operates totally grade-separated, Monorail vehicles can
accelerate at their maximum system acceleration, typically, about 3.0 MPH/s and
completely safely and easily achieve typical maximum operating speeds of 55 MPH
even in dense urban areas.
Imagine the dual-beam Monorail guideway as a
circle. The Monorail travels 20 miles south along one beam, then turns around
and travels North 20 miles to the beginning. If one train services this route
and the train travels 20 MPH, then the train will take 2 hours to return to its
starting point. The headway is 2 hours.
Two trains each traveling 20 MPH will cut the
headway to one hour, but the same one hour headway can be achieved by a single
train traveling at 40 MPH. Therefore, a fewer number of faster trains can
substitute for a larger number of slower trains. The passenger carrying capacity
is the same.
We now present a simple model showing the positive effects of
Monorail being able to achieve a faster maximum speed in dense urban areas.
The following assumptions were used:
System
Length: 20 miles of dual-beam guideway, with turn-around
Total
Number of
Stations: 14
Average
Distance Between Stations: 1.54 mi
Dwell
Time at Each Station: 30 sec.
Maximum
System Acceleration: 1.25 m/s**2 = 2.8 MPH/s
Desired
Headway: 10 Minutes
Passengers / Train: 224 (84 Seated, 140 standing, 2.7 ft**2/passenger).
Average
Number of Stops Each Passenger Rides: 6
This route is our 'Red' line which bypasses
IH-35 along Lamar, Guadalupe, Lavaca, South First, and South Congress. We assume
that the Monorail vehicle accelerates at 2.8 MPH/s until it reaches any one of
the following maximum speeds. It then decelerates at the same rate in time to
reach the next station, where it dwells for 30 seconds. Each passenger is
assumed to travel for 6 stops, which is approximately the number of stops from
one end of the line to the center, thus simulating a typical commute into the
central business district. We then examine the effects of maximum system speed
on average speed, average passenger trip time, the total number of trains
required and the total number of passengers per hour per direction (pphpd)
carried by the system.
Keep in mind as you read the chart above that
the national average light rail speed (FY2000) was 15.3 MPH in 2000 (source:
American Public Transportation Association:
Light Rail Summary Data,
FY2000.
This would put a light rail vehicle somewhere
in the left side of each graph below compared to the Monorail which we will
assume has a maximum system speed of 55 MPH.
This leads to the following conclusions:
