Travel Forecasting And Traffic Analysis
Travel Demand Forecasting • A process of determining what the travel conditions will be like in the future • Whether the improvements under consideration will be able to handle the future traffic within environmental constraints • Required by FHWA to qualify for federal funding
Travel Forecasting Division • Provides existing and future traffic forecasts and traffic analysisto all of SHA and other customers for • Pre-project Planning / Feasibility Studies • Planning • Purpose & Need Traffic & Analysis (Existing and Future No-Build ) • Engineering Traffic & Analysis (Build Alternatives Evaluation) • Environmental Traffic (Air and Noise Impact Analysis)
Travel Forecasting Division • HNI, CTP and HPMS • Title Sheet & Loadometer Data for Highway and Bridge Design • Before & After Studies for SHA projects • Review of Traffic Impact Reports • Miscellaneous Traffic Requests • MDOT and other modals, • MPOs and local agencies • Elected Officials, Focus Groups, Citizens
ADT, AADT, AWDT Average Daily Traffic (ADT)The average number of vehicles passing a specific point on a roadway during a 24 hour period over a certain period of time • ADT = Total Volume of Trafficfor N number of days N • Annual Average Daily Traffic (AADT) • AADT = Total Volume of Traffic for the entire year 365 Average Weekday Daily Traffic (AWDT)
ADT, AADT, AWDT Example: The following 24 hour traffic counts are taken on a highway segment Tuesday: 28,000 Wednesday: 32,000 Saturday: 24,000 What is the ADT ? 1/3 (28,000 + 32,000 + 24,000) = 28,000 What is the AWDT ? 1/2 (28,000 + 32,000) = 30,000
Peak Hour & Peak Period Volumes Peak hour volumes are hourly volumes in AM and PM rush hours. When peak hours extend into multiple hours, we have peak periods.
Peak Hour & Peak Period Volumes Note the peak hour spreading
Seasonal Variation • Variability in traffic volumes due to change in time of the year • Typically occurs on roads with heavy recreational traffic
Seasonal Variation Example: • 24 hour traffic counts taken for a similar seasonal facility in Jan and Aug 2008 show volumes of 16,000 and 28,000 respectively, what is the AADT ? Answer: AADT based of Jan 08 count = (28,000/139%) = 20,150 • AADT based of Aug 08 count • = (16,000/76%) = 20,150
How do we forecast future traffic ? What is in the infamous “Black Box”?
How do we forecast future traffic ? • Growth Factor Approaches • Travel Demand Models
Growth Factor Approach Past Trends/Historic GrowthTracking past growth trends on a highway system to extrapolate future traffic forecastsUseful in rural areas where there is small and steady growth in the population.
Growth Factor Approach Contd… Socio-economic /Land Use Projections Use of local zoning information, land use projections to develop future forecasts
Growth Factor Approach Contd… Example: What is the percent increase in HH? = (2030 HH – 2000 HH)/ 2000 HH = (2641-379)/379 = 5.96 ( approx: 600%)What is the growth rate of Employment?% Increase in EMP = (42649-10807)/10807 = 2.95 (approx: 300%) Growth Rate = 300%/30 years = 10% per year
Growth Factor Approach Contd… • More reasonable as it is based off of local inputs; however could present some inherent biases • A combination of traffic trend data and land use data results in better forecasts
Travel Demand Models A “model” is a simplified abstraction of reality ➲ Transportation models mathematically represent how choices are made when people travel ➲ Travel demand occurs as a result of individual travelers making decisions about how, when and where to travel Source: http://www.edf.org/documents/1859_InsideBlackBox.pdf
The Four Step Process Entire modeled area is broken down into TAZs Land Use Data Household Population Employment Projections in 5 year increments by TAZ Transportation Network Data All intersections/ crossovers as nodes All roadway segments as links All transit routes as paths Source: www.mwcog.org
Sample Land Use Data MWCOG Nodes & Links MWCOG TAZ Structure
Trip Generation How many trips will be there? • Study Area Subdivision into TAZ based on geographic boundaries, special generators • Socio-economic data • Demographics • Income • Vehicle Ownership • ---- • Land Use data • Physical Characteristics • Number/ Type of Units • ----
Trip Generation • Trip Productions • Trips beginning in each TAZ • Trip purposes: HBW, HBO, NHB • Trip Attractions • Trips ending in each TAZ • Destination choice models Source: NCHRP 365
Trip Generation • Trip Rates Based on Activity Unit • NCHRP 365 rates • ITE Trip Generation Manual • Cross Classification Tables • By Household Size, Income, Auto Owned Source: NCHRP 365
Trip Generation Where will the trips be? • Convert trips generated by zone into O-D pair travel pattern Trip distribution calculated as a function of “distance” between O-D pair
Trip Generation Many possible combinations • 10 zones -> 100 combinations • 1000 zones->1,000,000 possible • O-D pair combinations!! Zone 3 Zone 2 Zone2 Zone 3 Zone 1 Zone 4 Zone 6 Zone 5 Zone 7 Zone 8 Zone 9
Trip Generation Gravity Model • Analogous to Newton’s Law of Gravitation! • The number of trips between zones are directly proportional to the number of productions and attractions • Trips are inversely proportional to a function of the “friction” between zones measured in distance.
Mode Choice How will people travel? • The OD pair trips are split into modes • Auto: SOV, HOV, HOT, … • Transit: Bus, Metro, Tramway, … • Non-motorized: walk, bicycle, ...
Mode Choice How will people travel? • Requires lots of info about modes • Travel time • Cost • Convenience • Reliability • Quality of service
Traffic Assignments What routes will be used? • Traffic volume estimates on each link (ADT, AM, PM Periods) • Number of vehicles • Travel time (speed) • Aggregated results • Vehicle Miles of Travel (VMT) • Vehicle Hours Traveled (VHT)
Four Step Process Summary Source: www.mwcog.org
Purpose and Need Traffic EXISTING TRAFFIC • Study area limits are identified • Traffic database is inventoried to see existing counts in study area. • If needed, traffic counts are conducted throughout the study area (usually takes 2-4 weeks) • Types of count data : Classified, Turning Movement, Volume, ATR • Counts are factored to Average Daily Traffic (ADT) using Traffic Trends data from Automatic Traffic Recording (ATR) stations • AM/PM Peak Hour Turning Movements are developed
Existing Traffic Data RAW COUNT DATA TYPE OF TRAFFIC COUNTS : CLASSIFIED, TURNING MOVEMENT, VOLUME
Existing Traffic Data TRAFFIC TREND FACTORS TO OBTAIN AVERAGE DAILY TRAFFIC DATA Example: If a 24 hour class count on May 7 , 2008 (Wednesday) for an Urban Other facility shows 100,000 vehicles, what is the AADT? Answer: 0.89 * 100,000 = 89,000 If a 13 hour count on Nov 24, 2008 (Tuesday) for an Urban Other facility shows 10,000 vehicles, what is the AADT? Answer: 1.16 * 10,000 = 16,000
Purpose and Need Traffic Contd.. FUTURE NO-BUILD TRAFFIC • Future forecasts for design year are developed by considering proposed changes in • Land use • Transportation System (financially constrained) • Two approaches to develop Future No-Build traffic which is broadly based on study area • Growth Factor Approach (rural areas, non-MPO regions) • Travel Demand Modeling Approach (MPO regions)
Purpose and Need Traffic Contd.. Growth Factor based Forecasts • Typically used for rural areas where there is small and steady growth • Review historical trends and develop average growth rates • Review any zoning changes and add trips generated by new land use • Develop 20 + year forecasts
Purpose and Need Traffic Contd.. Travel Demand Model based Forecasts • Typically used in Metropolitan Planning Areas • Major Metropolitan Planning Organizations (MPOs) in MD • Metropolitan Washington Council of Governments • Baltimore Metropolitan Council • Wilmington Metropolitan Area Planning Council • MPO collects data on existing traffic, transportation improvements and land use • Develops a travel demand model for current and projected conditions
Purpose and Need Traffic Contd.. Perform traffic analyses for existing and future No-Build conditions • Level of Service (Volume/ Capacity) Analysis • Critical Lane Volume (CLV) based • Highway Capacity Manual (HCM) based • Micro-simulation based analysis • Synchro/ SimTraffic • CORSIM • VISSIM
Case Study Study Limits: East of MD 97 to west of I-95 , approximately 10.5 miles MD 28/ MD 198 Project Planning Study
MD 28/MD 198Land Use Summary Study area falls in the Metropolitan Washington Council of Governments (MWCOG) MPO region Travel Demand Model Version 2.1 D#50 Co-operative Land Use Forecast Round 7.0 Traffic Study Limits: MD 28/ MD 198 (MD 115 to Van Dusen Road)
Engineering Traffic/ Build Traffic Forecasts
Why is Build Traffic Needed? • To determine Capacity, which is the amount of traffic a facility can handle, with reasonable operational characteristics, over a given amount of time (typically an hour) • To determine Level of Service (LOS), a rating system for measuring the quality of traffic flow, which ranges from ‘A’ (excellent operating conditions) to ‘F’ (failing conditions)
No-Build vs. Build Traffic No-Build Traffic • Traffic projected to be on the road when no major construction takes place to change capacity. The traffic is for the most part based on future travel demand and operations with unchanged roadway conditions. Build Traffic • Traffic projected to be on the road when major construction does take place and changes in capacity occur. Roadway widening, lane additions, and bridge construction are good examples of physical changes that impact capacity.
No-Build AlternativeSAMPLE Intersection 300 Existing / No Build: 4 lanes on MD 300 313