1. Lecture 16 CE 2710 Norman Garrick
Capacity and Demand
Lecture 16
CE 2710
Norman Garrick

2. AADT versus Design Hour Volume
Often reports from state level traffic count studies give average annual daily traffic (AADT)
But we don’t use AADT for design, we need to get a design hour that correspond to a relatively busy hour of the day
The hour selected for design is referred to as the design hour
What hour should be selected?
Generally we select an hour that is any where from the 30th to the 100th busiest hour of the year
As a very rough guide the typical design hour volume is can be taken as about 10% of the AADT. But this % varies significantly depending on the temporal variation in traffic.
Once a design hour volume is determined then the designer must also determine the directional split at that hour
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3. Estimating Design Hour Volume from AADT
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4. Demand and Capacity for Street and Highway Design
Convention street and highway design is based on the idea of fitting capacity to demand Demand is characterized by a design hour volume Capacity is characterized by design hourly service volume
Norman W. Garrick

5. Demand
The design hour volume is meant to be the volume of traffic that will use the facility in the design hour, in the design direction, in the design year Usually the design hour is taken as the 30 busiest hour of the year DHV is often estimated from AADT
Norman W. Garrick

6. Estimating Traffic in the Design Year
In many projects, the DHV is based on traffic for 20 or 30 years in the future The procedure for doing this is some times derided as ‘predict and provide’ because in many cases it is based just on predicting past trends
Norman W. Garrick

7. Problems with Predict and Provide
Predict and provide is problematic because expanding capacity affects future demand. Providing increase capacity lead to more traffic volumes in the future than we would otherwise have. Over the last 60 years this has lead to a self re-enforcing cycle of increasing traffic. But, recent trends suggest that this cycle might be at an end.
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8. Traffic Trends in USA
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9. Traffic Trends in Connecticut Route 195 Data
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10. Predict and Provide in an Era of Decreasing Traffic
Decreasing or steady traffic volumes is one more reason to reject the concept of predict and provide Basing design decisions on a 20 or 30 year prediction of traffic volume is increasingly unacceptable
Norman W. Garrick

11. Consequences of Decreasing Traffic
Before beginning a $2.5 billion project to widen the New Jersey Turnpike, turnpike officials said the construction was necessary to reduce existing congestion and to cope with future traffic.
"Turnpike traffic is on the rise," the state Turnpike Authority said in its justification for the project. "By 2032 northbound traffic volume is expected to increase by nearly 68 percent [above 2005 levels]; southbound traffic is forecasted to increase by 92 percent.”
Now, one-third of the way through that 27-year forecast, turnpike traffic is actually about 10 percent lower than it was in 2005.
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12. Capacity How is the capacity of the Hoover Dam Determined?
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13. What is the Capacity of a Street?
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14. Understanding Capacity for Vehicle Travel
Determining vehicle capacity on a street is not really like determining the amount of water in a measuring jar
Capacity is not a fixed number – it is rather a number selected based on what level of congestion we are willing to put up with
The designer has flexibility in selecting a design capacity She does this by designating a Level of Service Once a LOS is determined then the design hourly service volume can be selected from a chart
Norman W. Garrick

15. LOS for Freeways LOS in urban areas is usually based on intersection flow
Norman W. Garrick

16. Capacity and the Level of Service
What are the trade-offs involved in selecting a low LOS?
Norman W. Garrick

17. Capacity and the Level of Service
Selecting a low LOS means that you are designing for a low level of congestion during the busiest hour of the year
That means the facility will be empty for most hours in the day
Some cities now require that we design for LOS E or F to reduce inefficiency and the impact on the urban area of having large facilities
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18. What is the Capacity of this Street?
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19. New View of Street Capacity
By Ian Lockwood
In the past, Street Capacity = Design Hour Service Volume Now, Street Capacity = Social Capacity + Economic Capacity + Travel Capacity
Norman W. Garrick

20. Sample Calculation 1

21. Design Hour Volume Calculation
A new urban street is estimated to have a ADT of 15,000 vehicles per day. The directional split is 60% to 40% in the peak direction. The distribution of traffic for each hour in the year is given below Solution: Use 30th hour as design hour % of AADT in 30th hour = 8.6% from graph Traffic flow in design hour in both direction = 0.086*15000 = 1290 Design Hour Volume in peak direction = 1290 * 0.6 = 774

22. Traffic Hourly Distribution
Hour volume
as % of ADT
Number of Hours
with traffic great than
shown

23. Number of Lanes Need one lane in each direction
A new urban street is estimated to have a ADT of 20,000 vehicles per day.
The directional split is 60% to 40% in the peak direction.
Determine number of lanes at a Level of Service of E
Solution:
Design Hour Volume in peak direction = 1290 * 0.6 = 774
Estimated Capacity for Level of Service E (the Design Hour Service Volume)
= 820 vehicles / hour / lane (from a chart)
Number of lanes needed = DHV/DHSV = 0.97
Need one lane in each direction