Quote:
Originally Posted by cwkimbro
You've provided a good start for looking at this, but the analysis assumes blanket assumptions and assumptions that stay constant instead of -variable- to the conditions that are present. Human behavior changes with conditions. You are also making a false assumption that the HOV lane capacity doesn't max out and get congested and if it doesn't that the HOV lane operates at peak capacity instead of under peak capacity.
Assume for a second that no one speeds...and the speed limit is 65mph. (just for the sake of simplifying the scenario and point... in practicality this assumption doesn't matter)
If three lanes are flowing at 65mph there is no incentive for anyone to use the lane unless...the price of the lane at that moment is $0.00 or they can use it for free via a hybrid system as a bus lane or a hyrbid as an HOV lane (+2 or +3, etc...). Some systems are hybrid HOV+2 and some are HOV+3 and a few don't allow HOV, but just transit. In the case of the I-85 N corridor it is HOV+3+transit. Previously the lane was HOV+2, but got congested and suffered the consequences of lowered capacity+slower transit. However, there are not enough HOV+3 cars to completely utilize the lane capacity effectively.
The early stages of congestion starts... In the three lanes traffic slows from 65 to 60. It's still really early, the price goes up but stays cheap. The price stays cheap enough that encourages cars to move to the HOT lane even just to go slightly faster than the general lanes, but expensive enough to prevent the HOT lane from being as congested as the general lane.
As conditions get worse in the general lanes, the price changes in the HOT lane to influence it to keep operating at peak capacity regardless of what the general lanes do. In other words the price goes up.
In other words ... you can't have a blanket assumption that the HOT lane discourages a fixed percentage of cars from using it.
--The lane itself is designed to have a variable pricing procedure to encourage/discourage the right amount of cars to keep the lane operating at peak capacity.--
The only conditions that would exists where you get a loss of capacity from the HOT lane is when the system is new and people need to sign up for a toll account, the system's variable price tolling strategy and computer system needs to be re-programmed to peoples behavior more efficiently, and/or the entry/exits need to implemented better. The first condition only exists as the HOT lane is started and the last two are conditions that can be engineered to make the system work effectively.
If you maintain HOV+2, the HOV+2 lane gets congested and capacity is lowered. HOV+3 will be used by too few cars. If you change it to HOV+3/HOT then capacity is maintained, which keeps cars moving and shortens congestion in the general purpose lanes.
Even if you widen the road to maintain HOV+2 without congestion, you can still sell excess capacity as a HOT lane that keeps capacity moving, but will pull more cars off the general lanes.
In the scenario with I-85N the only way conditions could get worse is under the following condition:
# of drivers that change from HOV+2 to SOV > Capacity increase from HOT service causing congested HOV+2 to be free flowing + increase transit ridership from quicker commuter bus.
But, there is an incentive that some (not all) HOV+2 riders will stay HOV+2, since they can use the HOT lane at half price (if split between the carpool). Another option I wish the I-85 North corridor would consider is offer a small discount for HOV+2 to influence 2-person carpools at staying 2-person carpools, but even as a carpool the lane can be managed/priced/behavior adjusted to maintain peak capacity.
The only scenario where HOV+2 (+3, etc..) can be better than the HOT lane is if there just happens to be the right number of HOV+2 at every given moment to keep the HOV+2 operating at peak capacity. However, this is extremely improbable as conditions change over time. Even if it operated at peak capacity at one point in time, 15 minutes later it could be below capacity or there won't be enough HOV+2 drivers to make full use of the space in the HOV+2 lane that a few SOV could use.
I know people are skeptical about the simple fact that it is a -toll-, but it is the only way for a system to manage a lane to operate at peak capacity (not above or lower) throughout the peak load time. This is because it is the only way to fluidly/variably influence human behavior at any given time.
Everyone has a price point at when they would get in the lane or get out.
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First I already acknowledged the issue of assumptions. I usually do them in order to avoid my natural tendency to calculate everything from every angle. At some point, a variable needs to be fixed in order for a conclusion to be presented.
A variable toll addresses the issue of capacity reduction, and I do feel that enough study would create a formula by where the demand for the lane is automatically fixed to the capacity of the lane.
I also like the idea of selling excess capacity to single occupancy vehicles when the double HOV isn't full. (My reasons for advocating a double HOV are because of the benefit to traffic flow when a car has the ability to pass a slower moving vehicle and the compound incentive to ride with someone else.)
Alright, I'm on board with that to a degree.
My goal is to end the concept of congestion through prudent planning as congestion is an economic and quality of life negative. In order to accomplish this, I'd like to increase capacity above what is needed during peak hours, and furthermore do it the cheapest way possible.
The benefit of grade separated rail is that it does not interact with congestion no matter the conditions. In the straddling bus, I saw the possibility of achieving the same, with some intelligent civil engineering, without the cost associated with heavy rail.
A peak capacity HOT with buses still has a few drawbacks to this, most notably the fact that the bus will have to exit the highway to stop and that a stop every, say 2 miles (i.e along the route as opposed to point to point), means that the bus really can't really access the HOT at all.
As for interactions between cars and the straddling bus, I've considered a thin, but effective physical barrier between the bus and the traffic. Of course, I never suggested that this come to Atlanta. I do, however, think that with a few test courses and some smart people, it can be achieved with a tangible benefit.
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Thank you very much!
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