Post by John Lansford Post by Dick Boyd
What are the major flaws of the US road system?
1. Poorly constructed surfaces. Tendency to hydroplane and cover it
up by posting a slippery when wet sign ...
I'm having a hard time coming up with a road flaw that isn't a people
flaw. So maybe the roads are in good shape but we can't say the same
about the people that operate them.
2. People that build and operate roads don't understand capacity.
They think congestion is a good thing and more places should be
Uhhh, right. As one of those "people who build and operate roads", I
most certainly do "understand capacity". So do the engineers who work
with me in this profession. Roads are designed to handle the expected
traffic 20-40 years into the future; sometimes conditions change
during that timeframe that causes the growth rate to greatly exceed
the projected volumes, however.
What you apparently don't understand about capacity is not every road
can be upgraded simultaneously or instantly, and that growth happens
faster than roads can be improved. As costs rise, fewer and fewer
road-miles can be improved for any given amount of money, so the state
must pick and choose where to put that money where it will do the most
John Lansford, PE
Obviously our understandings of capacity differ. I assume you are
going by the Highway Capacity Manual (HCM), ITE, AASHTO, MUTCD, AADT,
fluid flow and other Civil Engineering books in your understanding of
capacity. Maximum number of cars to pass a given point in a given
time. Level of Service? I'm certain you understand those terms since
you sign your name with a P.E.
But do you understand my concept of capacity?
My understanding of capacity is based on a two second rule. At 65 MPH,
if the spacing between my car and the car I am following is two
seconds and the spacing to the car following me is two seconds, the
road is at capacity. That figures out to less than 1,800 cars per lane
per hour in round numbers.
Why two seconds? My scan pattern and reaction time are comfortable at
two seconds. My personal equation, if you will. Get any closer behind
and I feel tailgated. Nothing the following driver can do to get me to
go faster short of contacting my bumper and pushing. In transition
areas, I'll change my scan and concentrate more on the lead car. First
thing I cut out of my scan is instruments. I no longer monitor the
speedometer. I'm not comfortable in not monitoring the speedometer for
minutes at a time. I no longer monitor oil pressure, or gas level, or
battery condition when spacing is less than one second. I stop
scanning the outside rear view mirrors, instead I concentrate on
staying in my lane and use the inside rear view mirror. If a car
encroaches into my lane at that level of congestion, there is no where
to go. Why should I cause a crash by trying to avoid a person that
encroaches on my lane? Very stressful driving above thirty MPH closer
than two seconds.
Other drivers may have a "capacity" number of one second, or two
hundred milliseconds, or five seconds. A two hundred millisecond
driver following me makes me nervous. I find myself spending more of
my scan time on the tailgaiter and in the process lose my spacing on
the car ahead of me.
Drivers have a spread of capacities. Below thirty MPH, I might accept
a one second rule. The road instantly doubles in capacity, but my trip
time also doubles. Pulling away from a traffic light, I might accept a
zero time. That is I'll start rolling the same time the car ahead of
me moves. Only I won't accelerate as much.
Do you, as an engineer, build the road for me, as a driver? Or do you
build the road for your engineering peers? Who pays your salary and
where do they get the money to pay you? Is the customer always right
at your shop? Or only when he/she uses the Highway Capacity Manual
My understanding of HCM definition of capacity is that flow at
capacity is unstable. As more cars try to come into the flow, drivers
interact more with each other and to compensate for the increased scan
workload, will slow down. Add, or try to add, one more car to the flow
at capacity and the whole thing comes to a standstill. Richard
Haberman (Mathematical Models), Haight, Eno, Gazis, Whitham, or
Berkely researchers explain the flow models so even a high school math
teacher can understand them.
By my definition of capacity, flow will begin to slow below the posted
speed on the addition of one more car. Take away that one more car and
you have my definition of capacity. Capacity, to me, implies the
ability to travel at the posted speed for the entire trip. For city
streets with traffic signals that means no stops. The lights are
synched and platoons travel at posted speeds. I beleive the HCM calls
this LOS A.
In my understanding, the capacity of a road can be doubled instantly.
Measure the number of people carried instead of merely the number of
vehicles. Join a car pool. With fewer vehicles, the road does not
Shirley Highway reversible lanes are a case in point. The
Transportation Board and some engineers insist on operating the
reversible lanes "at (vehicle)capacity". Meaning 45 MPH speeds on a
posted 65 MPH that can be safely driven at higher speeds using my
(people)definition of capacity.
Lomax at Texas Transportation Institute, author of the Congestion
studies is on record that congestion is a good indicator for an area.
Congestion represents a good economy.
Using AADT (Average Annual Daily Traffic) masks the benefits of
shaving peak loads. Los Angeles is just coming around to scheduling
truck traffic in and out of the port so it does not conflict with
commuters. AADT will soar. Congestion may even drop.
I was a little harsh in the way I stated the problem as I see it. Let
me restate my view.
The major flaw in the US road system is that the engineers, the
politicians, the drivers are all speaking a differnt language. They
use the same words, but assign different meanings to those words. One
man's capacity is another man's congestion. One man's highway trust
fund is another man's pork barrel.
The civil engineers also speak about late arrivals five percent of the
time as being acceptable. Not in my book. Civil Engineers suggest that
motorists compensate by planning trip times based on six sigma lower
speeds. So every trip has extra road time built in. Are civil
engineers speaking the same language as motorists?
Civil Engineers talk about average travel time, when in my book, the
variation from average is more important.
Civil Engineers even accept statistics such as fatalities per hundred
million miles and set goals to get the number below one. The goal
should be zero. Every crash fatality has some road engineering factor,
in my opinion.