Traffic Flow and Capacity for

Street and Highway Design

Lecture 5.1CE 4720 5720

Norman GarrickNorman W. Garrick

Norman W. Garrick

Travel Flow Data Travel Flow is not Travel Demand

Good travel flow data for all modes of travel is important for transportation design.

However, it is important to understand that travel flow data does not necessarily mean travel DEMAND.

Travel flow measure is a reflection of how people are currently using the transportation system as it is configured.

Norman W. Garrick

Travel Flow Data Travel Flow is not Travel Demand

If the transportation and land use system changes then the travel flow is also likely to change.

For transportation planning and design we need to estimated what the traffic flow will be after we have implemented out design

This task is made more difficult if we treat travel flow as if it was some immutable travel demand that is not affected by the changes being implemented

Norman W. Garrick

Travel Flow Data Some Basic Concepts

One of the challenges in working with is that travel flow is that it varies significantly in both time and space.

We often do not have data at a fine enough resolution to fully capture these variations.

It is important to understand the likely variations in order to effectively interpreter the available data

Temporal Variation in Traffic Flow

Norman W. Garrick

Temporal Variation in Traffic Flow

Traffic flow vary by time of day, day of week, month of year and from year to year.

The pattern of variation depends on the specific location. For example, the temporal variation of traffic in Storrs is likely to be different from that in Willimantic.

Monday to ThursdaySaturday

http://www.ptt.uni-duisburg.de/en/projekte/babnrw/daten/

Norman W. Garrick

Effect of Temporal Variation on Travel

Both streets carry 20,000 vehicles per day

Which street would have more severe congestion?

Norman W. Garrick

Temporal Variation in Traffic Flow

One solution that is some times used to reduce temporal variation is differential pricing.

For example, many transit systems charge a higher rate for travel before 10 am and after 3 pm. This helps encourage people that have flexible plans to delay their travel to the off peak time.

This is the same approach used on some toll roads where the plan is know as congestion pricing.

Norman W. Garrick

Spatial Variation in Traffic Flow(including Directional Variation)

Norman W. Garrick

Spatial Variation in Traffic Route 195 - Storrs

Storrs

WillimanticMile Zero

Norman W. Garrick

Spatial Variation in Traffic Volume on Route 195, Storrs

2006 ADT by Mileage

East Brook Mall/Big Y UConn/Storrs Center

Norman W. Garrick

Directional Distribution in Traffic Flow

Directional distribution is a measure of the difference in flowbetween the two directions of travel

Total traffic flow 4,500 per hr.

Going West3,500 per hr.

Going East1,000 per hr.

Direction Distribution to West3500/4500 = 7/9 = 0.78

Norman W. Garrick

Directional Distribution in Traffic FlowA Solution?

Reversible lanes in the middle to deal with a severe directional variation.

(I believe this is just conceptual – I don’t know of any example of reversible lanes implemented in this manner.)

It illustrate the point that heavy directional split can result in a very inefficient use of road space

Norman W. Garrick

Directional Split in Traffic FlowThe Zipper Machine

Norman W. Garrick

Directional Distribution

In many urban areas trips are mostly going towards the central business district in the morning and from the CBD in the evening. This means that the trains and the roads are sized to carry the peak direction flow. If the directional distribution is very lopsided then this is a very inefficient system since the lanes and the trains going away from the center will be virtually empty in the morning.

One argument for mixed land use is that it helps to cut down on this directional over balance. So if a train is connect two mixed use centers (such as downtown DC and Arlington, Virginia) the directional distribution will be more balanced resulting in more efficient use of the transportation system.

Norman W. Garrick

Where does travel flow counts come from?

State Counts of Vehicle Traffic

The DOT maintain a program for counting traffic on all state owned highways, roads and streets.

There are two different type of counts: permanent count stations and temporary count stations

Permanent Count Stations give the most complete coverage of temporal variation in traffic

Temporary count stations are much less reliable since they are put out for at most 48 hours – the state then use factors to estimate the average daily count.

Other source of traffic count data are counts from individual towns or from developers working on larger projects.

Norman W. Garrick

Where does travel flow counts come from?

Pedestrian, Bikes and Transit Counts

I know of no agencies that routinely count pedestrian traffic – this makes it harder to include pedestrian issues in transportation planning

A handful of cities in the country, including Portland, Davis and Cambridge have programs for counting bike traffic

Transit counts are readily available from transit agencies and national transit bodies

Bikeway MilesCyclists per Day

1991 2007

Portland (OR) Bike Count Program

http://www.streetsblog.org/wp-content/uploads/2007/09/portland_bike_counts.jpg

Norman W. Garrick

Shared Bikes, ParisA New Era for Bike Counts

http://networkedblogs.com/g0g87(from Sam Goater)

Norman W. Garrick

Bike Parking at Train Station, Amsterdam

Norman W. Garrick

What is the State Traffic Counts Used for?AADT

The state traffic count is used to estimate an average annual daily traffic (AADT)

The AADT is meant to represent the average traffic over all 365 days in the year. In other words, it is meant to be

Total Traffic in year / 365

This can be obtained relatively accurately from the permanent count stations.

From the temporary stations, this is more difficult. The count from the station (which is referred to as average daily traffic or ADT) is multiplied by seasonal and day of the week adjustment factor to get an estimated AADT.

Norman W. Garrick

Estimating Traffic Counts for Design

Vehicles/hr or AADT

Often reports from state level traffic count studies give average annual daily traffic (AADT)

Since hourly volume for the design hour is what is typically used for design it is left up to the designer to come up with a reasonable design hourly volume from the AADT

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 estimate the directional split.

Norman W. Garrick

ADT and Design Hour Volume – Rural Roads

Traffic Flow and Capacity for Street and Highway Design

Convention street and highway design is based on the idea of fitting capacity to traffic flow

Traffic Flow 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 called‘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 traffic flow.

Providing increase capacity can 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

Traffic Trends in Connecticut

Route 195 Data

Norman W. Garrick

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 being rejected as an unacceptable approach

Norman W. Garrick

CapacityHow is the capacity of the Hoover Dam

Determined?

Norman W. Garrick

What is the Capacity of a Street?

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 streets

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

New View of Street Capacity

In the past, Street Capacity = Design Hour Service Volume

Now, Street Capacity = Social Capacity + Economic Capacity + Travel Capacity

Norman W. Garrick

By Ian Lockwood

Sample Calculation 1

LOS, Volume CapacitySample Calculation

A two lane urban street has an ADT of 20,000 vehicles per day. Estimate what fraction of the year this street will operate at i) LOS E, ii) LOS D. Do this calculation for directional splits of 50% and 60%. The hourly volume distribution and the design hour service volumes are given on the following sheets.

Traffic Hourly Distribution

Hour volumeas % of ADT

Number of Hours with traffic greater thanshown

Design Hour Service Volume

Urban Streets with Frequent Signal Controlled Intersections

Source: http://www.dot.state.fl.us/planning/systems/sm/los/pdfs/lostables.pdf

Assumptions: > 4.5 Signalized intersection, 1.5 % heavy traffic, 12 % left turn, 12 % right turn

Level of Service Design Hour Service Volume

A

B

C 275

D 700

E 850

F >850

Sample Calculations 2

Ocean Springs – Biloxi Bridge

Biloxi – Gulfport Region

Ocean Springs – Biloxi Bridge Location

Design Hour Service Volume

Source: AASHTO 1990

Level of Service Controlled Access Highway

Design Hour Service Volume per Lane

Uncontrolled Access Highway

Design Hour Service Volume per Lane

A 700 700

B 1100 1100

C 1550 1400

D 1850 1780

E 2000 2000

F >2000 >2000

LOS, Volume CapacityOcean Springs – Biloxi Bridge

1. Assume that the current annual traffic volume on the Ocean Springs-Biloxi Bridge is 32,000 vehicles per day estimate the LOS during the 30th hour of the year.

2. What would be the annual traffic for a LOS of D during the 30th hour.

Assume that the directional distribution is 40:60 and the traffic distribution follows the average curve from AASTHO given on the following slide.

Why is this Road So Empty?

1. Predict and provide for 30 year in future2. Design for 30th busiest hour of year3. Design using a capacity based on low LOS

Predict and Provide

Based on this data Mississippi DOT estimated that traffic would increase at the rate of 3% per year for this project

Predict and Provide

This is what a 3% annual growth in traffic volume looks like

Predict and Provide

This is how the actual traffic volume compares up to 2011