CE 453 Lab #3Page 1 of 4

DESIGN TRAFFIC DETERMINATION

PROBLEM:

In the design of any street or highway the designer must select a design life for the facility. In the case of most highways the geometric design life is 30-60 years and the pavement design life is 20-30 years, taken from the date of construction. The length of the design life is affected by policies established before and during the project’s life for the environment in which it is placed. Residential and commercial development, geography, expected traffic, and soil conditions may affect the performance of the pavement and the operation of the facility. The objective of this laboratory exercise is to estimate the traffic volumes and percentage of cars and trucks that will use the US 20 facility over the course of the next thirty years. This estimate will be used in the design of the pavement thickness, completion of the environmental impact statement, and will affect the horizontal and vertical alignments.

CONSTRAINTS:

The design life is especially important in the design of the pavement thickness. We will assume a 30-year design life for the proposed facility (pavement). A traffic assignment or prediction for the design year (30 years from construction) is required. You will find that current traffic information is often limited. In this case you will find information on traffic volumes from the files you accessed in the GIS lab. In an actual situation you would expect to contact state and local officials for existing information and, if necessary, to conduct your own traffic counts at desired locations. You may also find that the owning agency (DOT or city) will provide the design life information or even the pavement design.

Traffic estimation is not an exact science. We cannot make an exact estimate of the design year traffic due to the fact that we cannot know the future along the route. Land uses may change along the route or in an adjacent area that result in added or reduced traffic along the route. Legislation can change the legal weights of vehicles using the route and the environmental (weather, soil) conditions can turn out to be different than expected during construction. You are expected to support your estimate of traffic during the design, construction, and life of the facility. It requires some thought and planning to make such an estimate.

COMPLETION STEPS:

  1. Determine the DOT’s “current” value of the AADT on the US 20 extension in HardinCounty, from the Iowa DOT’s Traffic Book. Access it at Use the pull-down menu under Transportation Data and select Volume of Traffic on the Primary Road System. Go to HardinCounty and select US 20 (use the value to the east of the US 65 interchange). Update these values to our assumed opening year (of the highway) of three years in the future using three growth scenarios, low (0.5% annual rate), medium (1.0%), and high (2.0%). You might want to copy the numbers from the Traffic Book and paste them into Excel, taking care to keep track of the headings for the numbers. If you do your work in Excel be sure to show any equations you use.
  2. Add development to the area around the intersection (future interchange?) at County Highway D35 (in Section 14, JacksonTownship). The new development will be a 500-thousand square foot shopping center in the northeast quadrant of the intersection, a “big box” lumber/hardware of 100-thousand square feet in the southeast quadrant, a specialty retail center of 25-thousand square feet (drive-up bank of 5,000 square feet, high-turnover restaurant of 5,000 square feet, and the balance retail) in the southwest quadrant, and a motel (100 rooms) and convenience store with (16) gas pumps in the northwest quadrant. For the purpose of your calculations assume these will open two years after the highway is completed or five years from now. Use the appropriate equations from the ITE Trip Generation Tables, provided in your resource notebooks. You will need to determine both ADT and peak hours, plus the directional splits of the traffic (entering and exiting).
  3. Pass-by trips are trips that would have been on the system and that stop at the new development, then continue on their way. For the purposes of traffic on the highway we do not add the pass-by trips to the base traffic. Account for these using the following values:
  • convenient stores – 60% (therefore only 40% of the traffic to the convenient store is new traffic to add to the totals)
  • banks – 15%
  • shopping centers – T = [(157.357/X) + 0.022]-1for PM peak hour trips, where X is the ADT on adjacent road
  • if not listed, consider that pass-by trips are not an issue
  • Although pass-by trips do not add to the total traffic, when you are doing the turning movement diagram for the intersection you do include all of the trips going to the development whether or not they are pass-by.
  1. Update the three scenarios for the highway base traffic to the opening year of the development, two years after the highway opens.
  2. Assume that the traffic from the development will only be allowed to access US 20 at the CH D-35 intersection. Proportion the development-basedtraffic east and west on US 20 in proportion to the populations of nearby cities to the east (Cedar Falls and Waterloo) and west (Fort Dodge, Webster City, and Iowa Falls).
  3. Using the same three traffic growth scenarios, develop estimates of the traffic on US 20 east and west of CH D-35 for the period from the opening of the development out to the 30-year design year in five-year increments. Be sure to combine both the base traffic and the traffic from the development.
  4. Develop a recommendation for which of the three growth scenariosis to be used in the design of the highway, taking into account historical data or other sources you may find. You have some latitude in this task but you must provide verifiable documentation for your data sources.
  5. Assign design year traffic to the intersection in proportion to the east and west flows on US 20. You will need to provide turning volumes for both the daily traffic and the two peak periods (AM and PM).
  6. Summarize the components of the design year traffic. The 30-year ADT demand volume is the sum of the componentsdeveloped in the previous steps. Round traffic volumes upward to the nearest 100 vehicles on each of the route subdivisions, as well as on all summaries and tables. Select the route segment with the highest value for your project. This is done to provide the data for the design of the pavement to meet the most critical situation. Based on design values for the design hourly volumes (assume 10% of daily traffic) and traffic mix (car and truck percentages – assume 8% trucks) calculate the appropriate values for the design section of roadway.
  7. If you do not feel you have enough information you may make assumptions or you may do additional research. In either case you must document what you did; if you make assumptions you should be prepared to explain or defend them.

You may find it easier to do the calculations if you set up an Excel spreadsheet with separate tables for each growth scenario. You should set up a sketch layout of the site to facilitate assigning the traffic to the various roads. As a check on your work, you should see an approximate range of values as follows for daily traffic in the design year:

  • Base traffic (without development) on the highway from 6,970 to 10,900
  • Mall traffic should range from 24,000 to 35,000
  • Traffic for the lumber/hardware (using the tables for free-standing discount store) from 6,400 to 9,200
  • Restaurant traffic from 700 to 900
  • Bank traffic from 1,200 to 1,800

FUTURE USES OF YOUR RESULTS

You will use the results of this lab in one or more future labs. Be sure that you keep your results; you may also wish to discuss the results with your instructor (acting in this respect as your client) to verify the values to use in these future labs.

GRADING CONSIDERATIONS AND EXPECTED RESULTS:

1. Current traffic assumptions and calculations

a. Tabulation of current traffic volumes and projections of the current traffic of three growth scenarios to “opening year”.

b. Graph of vehicle miles of travel on US 20 in corridor during the 30-year life span, based oncurrent traffic and three growth scenarios.

c. Map of land use with new development indicated

2. Development Traffic calculations and location presentation

a. Sketch of proposed route and relative development locations and your perception of how total trips will be distributed (development induced trips only).

b. Written assumptions for traffic distribution from the developments.

3. Design Traffic values

a. List the design year (opening year + 30 years) values for the three scenarios along US 20 to the east and west of County Highway D35(highest traffic volumes):

ADT for Design Year______

DHV (Design Hour Volume, based on 10% of the ADT)______

ADTT (Average Daily Truck Traffic, based on 8% of the ADT)______

b. Provide a graphic layout showing the AM and PM peak-hour turning volumes at the CH D35 and US 20 intersection. See the following figure for an example.

REPORT FORMAT AND GRADING CONSIDERATIONS

The report MUST be submitted with these section headings. Make sure that you cite all references used. Include a printed copy of your calculations’ spreadsheet in the appendix.

  1. Introduction and Location Map 10%
  2. Current Traffic 10%
  3. Traffic Growth 15%
  4. Development Traffic 15%
  5. Design Traffic and Turning Volumes, including graphic showing peak-hour turning volumes 15%
  6. Conclusions and Recommendations5%
  7. Grammar and Presentation, including showing all your calculations (formulas on spreadsheets) 30%