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Interpreting Demand and Capacity for Street and
Highway DesignLecture 5.1
CE 4720 5720Norman Garrick
Norman W. Garrick
Norman W. Garrick
Travel Flow Data Some Basic Concepts
Good travel flow data for all modes of travel is important for transportation design.
One of the challenges is that travel flow varies significantly in both space and time. 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
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
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
http://www.interstate-guide.com/images/i-077_va_aadt.gif
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 in Traffic FlowDirectional Variation
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.
Norman W. Garrick
Directional 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
Characterizing Traffic Counts
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 determine the directional split.
Norman W. Garrick
ADT and Design Hour Volume – Rural Roads
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
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 unacceptable
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 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
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 great 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
Ocean Springs – Biloxi BridgeADT Pre-Construction
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
Estimate the level of service for the Ocean Springs – Biloxi Bridge.
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