Capacity and DemandLecture 16

CE 2710Norman Garrick

Norman W. Garrick

AADT versus Design Hour Volume

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

Norman W. Garrick

Estimating Design Hour Volume from AADT

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

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

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

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.

Norman W. Garrick

Traffic Trends in USA

Norman W. Garrick

1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 20100

500,000,000,000

1,000,000,000,000

1,500,000,000,000

2,000,000,000,000

2,500,000,000,000

3,000,000,000,000

3,500,000,000,000

Traffic Trends in ConnecticutRoute 195 Data

Norman W. Garrick

1994 1996 1998 2000 2002 2004 2006 2008 2010 201210000

12000

14000

16000

South of South Eagleville Road Between South and North Eagleville

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

Consequences of Decreasing Traffic

http://articles.philly.com/2014-03-10/news/48055048_1_bridge-traffic-turnpike-traffic-pennsylvania-turnpike-

commission

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.

man W. Garrick

CapacityHow is the capacity of the Hoover Dam

Determined?

Norman W. Garrick

What is the Capacity of a Street?

Norman W. Garrick

Understanding Capacity for Vehicle Travel

The designer has flexibility in selecting a design capacityShe does this by designating a Level of ServiceOnce a LOS is determined then the design hourly service volume

can be selected from a chartNorman W. Garrick

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

LOS for

Freeways

LOS in urban areas is usually

based on intersection

flow

Norman W. Garrick

Capacity and the Level of Service

Norman W. Garrick

What are the trade-offs involved in selecting a low LOS?

Capacity and the Level of Service

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

Norman W. Garrick

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

What is the Capacity of this Street?

Norman W. Garrick

New View of Street Capacity

In the past, Street Capacity = Design Hour Service Volume

Now, Street Capacity = Social Capacity + Economic Capacity + Travel CapacityNorman W. Garrick

By Ian Lockwood

Sample Calculation 1

Design Hour Volume CalculationA 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 graphTraffic flow in design hour in both direction = 0.086*15000 = 1290 Design Hour Volume in peak direction = 1290 * 0.6 = 774

Traffic Hourly Distribution

Hour volumeas % of ADT

Number of Hours with traffic great thanshown

0 50 100 150 200 250 300 350 400 450 5000

0.02

0.04

0.06

0.08

0.1

0.12

Number of LanesA 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 = 774Estimated 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