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Old January 19th, 2014, 10:19 PM   #2361
sirfreelancealot
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Quote:
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What speed would maximize the saturation flow rate?
I've heard 50 mph but then motorways in the UK with Variable Speed Limits have limits lowered to 40 mph when there is congestion but this may be more about slowing traffic down before it gets to the end of a queue and preventing rear end shunts.

Faster traffic needs bigger gaps for safe breaking distances - think about the two second rule and how much difference the distance is when travelling for 2 seconds at 50 mph and at 70 mph. If bunched up traffic is travelling fast on a busy road and someone brakes a bit too hard this starts up a chain reaction of everyone breaking a bit harder and so on to a point where the traffic comes to a standstill. This is why traffic appears to come to a halt for no reason - the 'phantom jam' phenomenon.
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Old January 20th, 2014, 04:21 AM   #2362
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Quote:
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I've heard 50 mph but then motorways in the UK with Variable Speed Limits have limits lowered to 40 mph when there is congestion but this may be more about slowing traffic down before it gets to the end of a queue and preventing rear end shunts.

Faster traffic needs bigger gaps for safe breaking distances - think about the two second rule and how much difference the distance is when travelling for 2 seconds at 50 mph and at 70 mph. If bunched up traffic is travelling fast on a busy road and someone brakes a bit too hard this starts up a chain reaction of everyone breaking a bit harder and so on to a point where the traffic comes to a standstill. This is why traffic appears to come to a halt for no reason - the 'phantom jam' phenomenon.
WSDOT looked at throughput along I-405 Northbound in May of 2001. In this case, traffic flowing at roughly 45 MPH resulted in the highest throughput (Source: September, 2006 Gray Book publication):



It's important to note that the posted speed limit of I-405 was 60 MPH during the study. What would the maximum throughput of a highway with a posted speed of 80 mph be? I believe the maximum throughput would shift up as the posted speed increases.
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Old January 20th, 2014, 07:50 AM   #2363
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sorry wrong thread
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Old January 20th, 2014, 01:33 PM   #2364
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I've heard 50 mph but then motorways in the UK with Variable Speed Limits have limits lowered to 40 mph when there is congestion but this may be more about slowing traffic down before it gets to the end of a queue and preventing rear end shunts.
Not rear end shunts (which can be avoided by using eyes - sightlines are good on the M25, even at 70mph).

When the limit goes down to 40, there's a fairly big 'sheer weight of traffic' (ie one or two people had to brake suddenly, those behind them had to do the same, and you've got a wave of braking travelling along the motorway, getting bigger and bigger) jam close ahead and if you go faster, you'll get to it before it dissipates*.

If it's a smaller jam, and/or further away, then the limit runs as 50mph. Also when they realise that there's a considerable amount of traffic they have it at 50.

*Which means that VSL has the annoying side-effect of looking pointless when it does its job.
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Old January 20th, 2014, 04:52 PM   #2365
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Quote:
Originally Posted by tradephoric View Post

WSDOT looked at throughput along I-405 Northbound in May of 2001. In this case, traffic flowing at roughly 45 MPH resulted in the highest throughput (Source: September, 2006 Gray Book publication):



It's important to note that the posted speed limit of I-405 was 60 MPH during the study. What would the maximum throughput of a highway with a posted speed of 80 mph be? I believe the maximum throughput would shift up as the posted speed increases.
No, the maximum throughput is related to operating speed, i.e. the gap between vehicles. Higher speed = larger gaps = lower throughput
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Old January 20th, 2014, 09:36 PM   #2366
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No, the maximum throughput is related to operating speed, i.e. the gap between vehicles. Higher speed = larger gaps = lower throughput
The 'two-second rule' is a rule of thumb by which a driver may maintain a safe following distance at any speed. Theoretically, if every driver on the road followed the '2 second rule', the throughput would be 1800 veh/hr/ln no matter what the speed limit is. The following chart taken from the 2000 HCM indicate that as speed increases, the flow rate increases:

This suggest that drivers deviate from the '2 second rule' as speed increases. There is 1.63 seconds between vehicles (3600/2200) when traveling at 60 mph. On the other hand, at 45 mph there is 1.89 seconds between vehicles (3600/1900).
Higher speed = less time between vehicles = higher throughput.
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Old January 20th, 2014, 09:43 PM   #2367
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The 2 second rule is textbook, but field research in the Netherlands has shown people drive much closer to each other, sometimes under 1 second, not only at 55 mph, but also at 70 - 75 miles per hour. This results in higher throughputs than what is theoretically possible, there's the Coen Tunnel in Amsterdam where they have recorded as much as 3000 vehicles per hour / lane before it was expanded.

Note that many six-lane freeways were designed for some 70,000 -80,000 vehicles per day back in the 1960s, but carry over 120,000 today, breaking with some of the previous theory on highway throughput.
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Old January 20th, 2014, 10:04 PM   #2368
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This results in higher throughputs than what is theoretically possible, there's the Coen Tunnel in Amsterdam where they have recorded as much as 3000 vehicles per hour / lane before it was expanded.
Damn! 3000 vehicles/hour/lane. What would be interesting to know is what was the average speed of traffic when they recorded the 3,000 vehicles per hour/lane in the Coen Tunnel (not what the posted speed is, but how fast was traffic actually traveling).
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Old January 20th, 2014, 10:22 PM   #2369
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I can't tell you that, but I do know the truck share is relatively low (10%) and that the Dutch drive fairly small cars, mostly in the city car - subcompact - small family car range. It's common to see trains of vehicles where people look through the next few cars to maintain such a short < 1 second distance.

The Dutch are notorious for keeping short following distances. Even on non-congested highways following distances of 1 second are rather common.
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Old January 21st, 2014, 12:10 AM   #2370
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Regardless of what the posted speed limit is along a road, some drivers are going to drive above than the speed limit, some drivers are going to drive below the speed limit, and some drivers are going to drive the speed limit. The distribution might look something like this under free-flowing-conditions:

~ 15% of drivers drive faster than the speed limit.
~ 15% of drivers drive slower than the speed limit.
~ 70% of drivers drive the speed limit.



I believe it would be impossible to maximize throughput along a roadway when the average speed is at or above the posted speed limit. This is because a percentage of drivers are going to drive below the posted speed limit and lead to gaps with faster moving traffic (slow drivers almost act as pylons and conflict with faster moving traffic). If the average speed of traffic slows down to the speed of the slowest drivers on the road, then maximizing throughput would be possible. As an exaggerated example, just look at the tight pack that forms along the backstretch at Talladega when all drivers are going the same speed... you can't maximize throughput much more than that!



The speed of the slowest drivers along an 80 MPH highway is going to be greater than the speed of the slowest drivers along a 60 MPH highway. That’s why I think the speed that maximizes throughput increases as the posted speed increases (put another way, speed that maximizes throughput would be achieved at roughly 85% of the posted speed limit).
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Old January 21st, 2014, 01:09 AM   #2371
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Pretty sure I've seen that on I-95 a few times, Big One and everything
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Old January 21st, 2014, 02:00 AM   #2372
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Quote:
Originally Posted by tradephoric View Post

The 'two-second rule' is a rule of thumb by which a driver may maintain a safe following distance at any speed. Theoretically, if every driver on the road followed the '2 second rule', the throughput would be 1800 veh/hr/ln no matter what the speed limit is. The following chart taken from the 2000 HCM indicate that as speed increases, the flow rate increases:

This suggest that drivers deviate from the '2 second rule' as speed increases. There is 1.63 seconds between vehicles (3600/2200) when traveling at 60 mph. On the other hand, at 45 mph there is 1.89 seconds between vehicles (3600/1900).
Higher speed = less time between vehicles = higher throughput.
You need to convert the time gap to a distance gap first. 2 secs gap is not the same at different speeds. The distance gap impacts how many vehicles can physically traverse a road section, different to theoretical vehicles passing a single point.

The higher peak throughputs occur at lower operating speeds and higher daily throughputs generally occur due to peak spreading.
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Old January 21st, 2014, 02:02 AM   #2373
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I-95 is really weird. I have driven on there in three lanes of traffic all driving about 85 or 90 mph with following distance of about 1 second.

But then, I was in heavy traffic in Germany between Cottbus and Berlin, in thick traffic with a Fiat van in front and a Skoda behind me, doing about 110 to 120 mph and following distance probably also 1 second (and right lane was just trucks and grannies, zoom zoom)
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Old January 21st, 2014, 05:34 AM   #2374
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You need to convert the time gap to a distance gap first. 2 secs gap is not the same at different speeds. The distance gap impacts how many vehicles can physically traverse a road section, different to theoretical vehicles passing a single point.

The higher peak throughputs occur at lower operating speeds and higher daily throughputs generally occur due to peak spreading.
The confusion might be how ‘throughput’ is being defined. In the May 2001 study, WSDOT referred to throughput as a volume of flow. They were looking at the flow rate of vehicles passing over a single point along the roadway (measurement at a point), and not an entire road segment.

Quote:
Page 61 of WSDOT Gray Notebook, Sept. 2006 (paragraphs directly before the 'relating speed and volume' chart):

Congestion not only causes delay, it also causes lost productivity for the roadway system. That is, under congested conditions, even though the road is “full” of cars, they are moving so slowly that fewer vehicles actually pass any given point on the road.Typically, the maximum throughput of vehicles on a freeway, about 2,000 vehicles per lane per hour, occurs at speeds of 42-51 mph, or about 70%-85% of the posted speed. The goal is to manage the system to achieve maximum throughput/productivity

As demand increases, congestion causes a drop in speeds. For a typical freeway, when speed drops to below 42 mph, or about 70% of 60 mph, the productivity of the freeway starts to decline. When congestion causes drivers to lower vehicle speeds to 30 mph, the throughput (volume of flow) on a freeway may fall from 2,000 vehicles per lane per hour to as low as 700.

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Old January 21st, 2014, 06:02 AM   #2375
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700 vehicles per hour on a freeway lane is incredibly low. This could only be a result of a phantom jam
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Old January 28th, 2014, 09:01 PM   #2376
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US 331, Florida

FDOT Breaks Ground on U.S. 331 Expansion Projects

Florida Department of Transportation (FDOT) Secretary Ananth Prasad, FDOT District Three Secretary Tommy Barfield, Florida Senate President Don Gaetz and Walton County Commission Chair Bill Chapman broke ground today on a new U.S. 331 bridge across Choctawhatchee Bay in Walton County.

Work will begin in January 2014 to construct a new $118.5 million bridge east of the current Clyde B. Wells Bridge and feature two northbound travel lanes with accompanying shoulders. The new bridge is scheduled to be ready for traffic in summer 2015, with final completion of the project set for a year later. Upon completion of the new bridge, the current structure will be used exclusively for two lanes of southbound traffic.

FDOT also broke ground today on the expansion of U.S. 331 from north of State Road (S.R.) 20 to I-10. The $47 million project includes widening the roadway to four lanes, safety upgrades, drainage improvements and new signage and pavement markings. Construction will begin fall 2014 with completion scheduled for summer 2016.

Construction on the 4.6-mile section of U.S. 331 in Walton County to multi-lane the roadway from the north end of the Clyde B. Wells Bridge to south of S.R. 20 began in September 2012 and is scheduled for completion in summer 2015.
December 20, 2013 press release: http://www.dot.state.fl.us/agencyres...ndbreaking.pdf

That's a pretty big project for the Florida Panhandle. It will effectively turn US 331 into a four-lane highway all the way from the coast to I-10 and twin the bridge across the Choctawhatchee Bay. The bay crossing is around 3 miles long, but not all of it is actually a bridge.
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Old January 29th, 2014, 09:06 PM   #2377
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US 17, North Carolina

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Coastal NCDOT snow response by NCDOTcommunications, on Flickr
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Old February 1st, 2014, 03:50 PM   #2378
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Richardson Highway, Alaska

A large avalanche blocked the Richardson Highway near the Keystone Canyon in Alaska, and cut off the coastal town of Valdez from the rest of the state. The avalanche created a lake behind it, flooding the valley and the road.

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rather impressive aerial video:


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Old February 1st, 2014, 05:00 PM   #2379
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Very impressive. Thanks for sharing ChrisZwolle. Nature is implacable.
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Old February 5th, 2014, 12:19 AM   #2380
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New Orleans, late 1950s

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New Mississippi River Bridge by CuteTeaBunny, on Flickr
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