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Re: No more roads...Where did you get your ramp meter dope, it's not entirely true...
Author: FUD
Date: 06-14-2019 - 21:39
Any conduit can have more capacity when it's bigger. Think roads (#lanes), railroads (#tracks), pipes/wires/rivers/canals (size), etc. If more capacity is needed, you can either make it bigger or control the flow into congested locations. Making it bigger runs into cost-effectiveness constraints; it costs money to build freeways, railroads, passing lanes, and the like, and for roads at least, in major urban areas, there's nearly infinite demand so added lanes will fill up within a few months to years. Once they fill up again, you just have more cars in the parking lot emitting fumes. So it's all a balancing act.
Ramp meters are common in California. They're not for safety per se; as Bob says, they're to manage the onramp flow so you don't get a lot of interruptions of the main line traffic - the main line moves more smoothly, and better uses its designed capacity.
If you like the plumbing analogy, think of rivers. Rapids occur where the flow is restricted by something like rockslides, bars, narrow channels, and large flows from a tributary. Usually, there's a stretch of smooth, slower water upstream of the rapids - the restriction actually is a natural dam. Then it squeezes through or over the bad spot and resumes normal flow below. A ramp meter is like a dam on a tributary that meters the flow into the river so it doesn't cause a backup - the river's flow is smoother if you do that.
The old Level of Service idea for roads tried to classify how a road operates at various traffic densities. A - hardly anybody around, drive as fast as you want. B through D - increasing amounts of traffic limiting what you can do, and from C to F speeds drop. E is at theoretical capacity, on a freeway usually around 30 mph, but is very unstable and can't be maintained very long - it drops to F (below capacity, stop n go) then picks up to D (near capacity but trudging along), rinse and repeat. Trains don't do that much because they have dispatchers, but we all remember those streetcar pictures in downtowns where they're bumper to bumper for blocks; it can happen.
Ramp meters are common in California. They're not for safety per se; as Bob says, they're to manage the onramp flow so you don't get a lot of interruptions of the main line traffic - the main line moves more smoothly, and better uses its designed capacity.
If you like the plumbing analogy, think of rivers. Rapids occur where the flow is restricted by something like rockslides, bars, narrow channels, and large flows from a tributary. Usually, there's a stretch of smooth, slower water upstream of the rapids - the restriction actually is a natural dam. Then it squeezes through or over the bad spot and resumes normal flow below. A ramp meter is like a dam on a tributary that meters the flow into the river so it doesn't cause a backup - the river's flow is smoother if you do that.
The old Level of Service idea for roads tried to classify how a road operates at various traffic densities. A - hardly anybody around, drive as fast as you want. B through D - increasing amounts of traffic limiting what you can do, and from C to F speeds drop. E is at theoretical capacity, on a freeway usually around 30 mph, but is very unstable and can't be maintained very long - it drops to F (below capacity, stop n go) then picks up to D (near capacity but trudging along), rinse and repeat. Trains don't do that much because they have dispatchers, but we all remember those streetcar pictures in downtowns where they're bumper to bumper for blocks; it can happen.
