Chris Stellar wrote:
There's real science behind freeway ramp metering, as I understand it‹the
semaphore-type signals (modeled on good old city stoplights) provide a steady but
variable drip of traffic so highways don't come to a standstill(at least not as often).
But in rush hours I've seen shorter city on-ramps back up more quickly than
their more roomy suburban counterparts, and the surrounding urban environment
can't handle the overflow as easily as more sprawlingly built areas can.
Sometimes that means multiblock gridlock in the neighborhood, with queueing
cars blocking thoroughfares, bike lanes, crosswalks, alleys and even
intersections.
Is this just an occasional occurrence in Minneapolis or is it a regular side
effect of city ramp meters? Perhaps the flow rate at our on-ramps needs to take
into account the system's unintended urban consequences.
Peter Vevang writes:
As I understand it, there are 2 reasons to meter the ramps. The first is to space out traffic so that it can merge properly. The second reason to meter ramps is that the traffic levels on the highway are beyond capacity, and incoming traffic is held off the highway by the meters to allow congestion to alleviate. Basically a holding area is created on the onramps, some people call it an artificial traffic jam. The artificial traffic jam was what Dick Day at the legislature went after and MNDot has mostly done away with that concept as I understand it. They found that the meters didn't improve commute times by holding people back for long periods of time.
Traffic meters help people merge better, so you don't have big rafts of cars
coming off of a red light hitting a highway all at once. But the overall
problem is vastly bigger than what a meter can affect. Put simply, there are
more cars than roads. A highway has a maximum lane capacity of 2200 cars an
hour, that is roughly one car every 2 seconds. Obviously the cars coming off
the onramp can't take up that full capacity because the lane is already mostly
full and they can't safely merge in high speed traffic. Imagine the number of
cars that are disgorged from the DT ramps on an average day, they all line up
at a few key locations to get out of the city and get dumped onto the highway
in a few short hours. The problem is basic math. And if we lower the safety
factor to allow more cars to merge by speeding up the metered lights, that
means more accidents. We can work the problem around the edges and fix a few
bottlenecks, but the bottom line is that as long as you presume that the
current safety factor for the meters is OK, you can't merge more cars without
significant infrastructure improvements. It seems like an impossible problem.
The meters are a symptom of a larger problem with the way we plan. I don't
think highway planners think about the city problems really. That isn't their
job, they calculate what the highways need almost exclusively, based on
principles of physics and various other factors, they hand all that other misc.
stuff off to other disciplines and units of government to handle. That is the
biggest problem with highway science, it is entirely geared towards highways,
everything is viewed from that narrow lens and it absolutely dictates the
planning processes for everything else. If I had my way, I would roll highway
planning into a larger state planning agency that could interact better with
cities and counties. Right now they are a hammer and everything looks like a
nail. You wonder why they make such ridiculous and sometimes bizarre decisions
some times, but from their perspective of traffic flow it always makes sense
and is entirely logical down to the last detail, and they are used to getting
their way. The core of highway science and planning hasn't changed
philosophically since the 1950's, that is why a highway today looks remarkably
similar to highways that went up from that era. We need more broadly based
design for our cities.
Maybe we need to start thinking outside the highway box. One radical solution
might be to get rid of the meters, AND the highway. We could turn 94, 394 and
35W into Parkways during rush hour with a 40 mph max speed limit until drivers
are out of the city. Driving slower increases highway capacity by a
significant amount of cars per hour and makes merging safer. If the speed
limit was that low you could also safely add 2 more lanes by taking up the
shoulders as car lanes and increase capacity even more. You could virtually
eliminate the merging concerns because large numbers of cars could safely merge
at that speed.
If we were to get fanciful we could take this one step further. We could
permanently drop the speed limit to 40 or 30 mph. This would also lower noise
levels and improve property values. You wouldn't need such large highway
easments on the sides of the highways. That land sits idle right now. There
are several hundred acres of highway easements near downtown alone, probably
worth a billion or more dollars if it was all developed. We are already
driving 30 mph or less during rush hour and we could use a $billion or two
right about now.
Peter Vevang
NE Minneapolis
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