Traffic Flow and Capacity for Street and Highway Design Lecture 5.1 CE Norman 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

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 Storrs Storrs Willimantic Mile Zero

Norman W. Garrick Spatial Variation in Traffic Volume on Route 195, Storrs 2006 ADT by Mileage East Brook Mall/Big YUConn/Storrs Center

Norman W. Garrick Directional Distribution in Traffic Flow Directional distribution is a measure of the difference in flow between the two directions of travel Total traffic flow 4,500 per hr. Going West 3,500 per hr. Going East 1,000 per hr. Direction Distribution to West 3500/4500 = 7/9 = 0.78

Norman W. Garrick Directional Distribution in Traffic Flow A 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 Flow The 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 Portland (OR) Bike Count Program

Norman W. Garrick Shared Bikes, Paris A New Era for Bike Counts (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

Capacity How 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 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 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 Capacity Sample 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 volume as % of ADT Number of Hours with traffic greater than shown

Design Hour Service Volume Urban Streets with Frequent Signal Controlled Intersections Source : Assumptions: > 4.5 Signalized intersection, 1.5 % heavy traffic, 12 % left turn, 12 % right turn Level of ServiceDesign Hour Service Volume A B C275 D700 E850 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 ServiceControlled Access Highway Design Hour Service Volume per Lane Uncontrolled Access Highway Design Hour Service Volume per Lane A700 B1100 C D E2000 F>2000

LOS, Volume Capacity Ocean 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 30 th hour of the year. 2.What would be the annual traffic for a LOS of D during the 30 th 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 future 2.Design for 30th busiest hour of year 3.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