Data Assembly

When a travel demand model is not available, the following items should be assembled when available and checked for their applicability for preparing a Project Traffic Forecast:

  1. Mapping or other roadway location drawings of the facility requiring traffic projections (Project Location Map).
  2. Graphical representation of existing lane configuration (i.e., straight line diagram (SLD), aerial photography, intersection diagrams, etc.).
  3. Data needed to determine traffic growth trends
    1. Historical traffic count data (current year plus nine (9) earlier years of mainline traffic data is preferred; but if ten years of data is not available, current year plus four (4) or more earlier years of mainline traffic and/or intersection approach volumes)
    2. Existing and future land uses which contribute traffic that would use the proposed facility
  4. Traffic factors
    1. K – Selected K Factor from the acceptable Standard K Factor Range
    2. D – This factor can be derived from one of the following sources: an FDOT count station in or near the study area where a Synopsis Report is available or a 24-hour to 72-hour project specific classification count taken within the project limits.
    3. T – The T factor can be obtained from either an FDOT Classification Station in or near the study area or a 24-hour to 72-hour project specific classification count taken within the project limits.
  5. The project opening and design years
  6. Current and historical population data within the study area
  7. Current and historical employment data within the study area
  8. Local Government Comprehensive Plan (land use and traffic circulation elements)
  9. Adopted MPO/TPO/TPA Long Range Transportation Plans (LRTPs)
  10. Current Transportation Improvement Program (TIP) from the relevant MPOs
  11. Current version of the FDOT Trend Analysis Tooll

Establish Traffic Growth Trend

There are several methodologies that can be used to determine the growth pattern, which include Linear Growth, Exponential Growth and Decaying Exponential Growth.

5.4.2.1 Linear Growth

Linear growth predicts the future traffic based on a straight line developed from historic traffic growth as shown in Equation 5-1. This method assumes a constant amount of growth in each year and does not consider a capacity restraint.

The equation for Linear growth is as follows:

$$Volume_{FY}=Volume_{BY}\times{(1+(FY-BY))\times{G_{Linear}}}$$
Equation 5-1

Where:

\(G_{Linear}\) = Linear growth rate
\(FY\) = Future Year
\(BY\) = Base Year

5.4.2.2 Exponential Growth

Exponential growth predicts future traffic based on a percentage of growth from the previous year as shown in Equation 5-2. This method is most suitable where there is rapid growth with no capacity restraints.

The equation for Exponential Growth is as follows:

$$Volume_{FY}=Volume_{BY}\times(1+G_r)^{(FY-BY)}$$
Equation 5-2

Where:

\(G_r\) = Geometric growth rate
\(FY\) = Future Year
\(BY\) = Base Year

5.4.2.3 Decaying Exponential Growth

Decaying Exponential growth is used to project future traffic in areas with a declining rate of growth over the analysis period, as shown in Equation 5-3. This method is recommended for site impact analysis in mature areas when build-out is approaching.

The equation for Decaying Exponential Growth is as follows:

$$Volume_{FY}=Volume_{BY}\times{\sum^{FY}_{BY}\frac{X}{FY-BY}}{\sum^{FY}_{BY}\frac{X}{FY-BY}}$$
Equation 5-3

Where:

\(X\) = Normal straight-line growth from trend data
\(FY\) = Future Year
\(BY\) = Base Year

The use of a particular growth pattern depends on the “goodness-of-fit,” as measured by the R-Squared value based on the regression analysis of the historical data. If historical count data are insufficient, prepare a similar analysis using alternative indicators such as population, employment, or other socioeconomic factors that influence travel decisions.

It is important to check for outliers in historical data before performing a linear regression analysis. Outliers are data points that deviate significantly from the rest of the data and can affect the accuracy and validity of the regression model. Traffic counts collected during special events or unusual times that do not represent normal traffic patterns, such as those during severe weather conditions or other unexpected interruptions, should be carefully evaluated before including them in the analysis. There are different methods to detect outliers, such as using boxplot or a scatterplot to identify outliers visually, or conducting statistical tests to quantify the degree of deviation of each data from the rest of the data. The outliers can be removed from the data, but it should be done with caution and justification, as it may introduce bias and reduce variability in the data.

In some cases, the regression analysis may result in negative growth. Unless there is strong evidence to indicate that the area will continue to experience a decline in economic activities or travel demand in the future, use of negative growth is not recommended. Under these circumstances, the project team should consult with the District or relevant local government agencies to assess the possible future growth patterns, and determine a reasonable growth rate. Some Districts have established minimum acceptable growth rates, and the project team should follow the District practices when developing the growth rates.

IMPORTANT NOTE:
It is important to check for outliers in historical data before performing trend analysis.

Develop Preliminary Traffic Projection

After growth patterns are analyzed and growth rates are established, future year traffic volumes can be computed by using the empirically derived growth trend equations. Alternatively, the FDOT Trend Analysis Tool has built-in formulae that perform the calculations automatically.

Check Traffic Forecast for Consistency and Reasonableness

  1. If future year geometric and traffic control design characteristics are firmly established (i.e., fixed by adopted plan(s) or constraints) determine the future capacity of the roadway section. If design is flexible enough to satisfy unconstrained demand, skip to #3.
  2. Compare the projected demand traffic volume to the available capacity. A constrained volume may be given, instead of an unattainable volume (e.g., a four-lane facility is 15 percent over capacity today and the project is for a six-lane facility, with trend analysis projections exceeding capacity for a six-lane facility). It should be noted in the Project Traffic Forecasting Report that the facility being designed will not be adequate for a 20-year design period.
  3. Review expected land use changes in the vicinity and determine whether projected traffic growth is consistent with the projected growth of population, employment or other variables and adjust if necessary. For example, if a new shopping center, office park, tourist attraction, etc., is expected to be built prior to the design year, then projections based on historical traffic trends would underestimate the design year traffic. In such cases, ITE trip generation rates could be used to establish daily and peak hour trips for the new land uses. A logical distribution of resulting site generated trips to available roadways should be based on knowledge of local travel patterns and used to adjust the traffic forecast. Conversely, the closing of an existing traffic generator would be expected to cause a reduction of the traffic forecast.

Develop Project Traffic Forecast Detail

  1. If the subject roadway intersection exists, use observed daily turning movement percentages at existing intersection(s) to convert future year link volumes to turning movement forecasts. Otherwise, logical turning movement percentages must be derived from observation of other roadways located in similar environments and/or specialized software that will calculate turning percentages utilizing the approach volumes. Note that the observed turning percentages are valid for future year forecasts only if land use and transportation network characteristics remain constant or if projected changes in those characteristics are proportional to the existing pattern.
  2. Review daily turning movements for consistency with special traffic generators, and transportation network characteristics in the vicinity. Use the ITE generation and logical trip distribution approach to adjust, if necessary.
  3. Balance adjusted daily turning movement volumes to achieve directional symmetry. A simple way to do this is to sum the opposing traffic movements and divided by two (2). There may be some situations when balancing the intersection may not be appropriate. See Chapter 7 for a more detailed discussion about estimating intersection turning movements.
  4. Use K and D factors to develop directional design hour traffic projections in the peak periods. The AM and PM forecasts usually involve reversing the peak direction of flow.
  5. Review the AM and PM design hour volumes for consistency with the trip generation activity pattern of the projected land uses in the vicinity and adjust volumes as necessary. Such adjustments are made with reference to observed differences in travel characteristics such as numbers of trips and directional splits that occur during morning and evening peak periods. Directional traffic counts collected at local land use sites may provide the necessary data or the ITE Trip Generation Manual may be used to obtain the peak period trip generation characteristics of various land use/special generator sites.

Analysis of Projections

  1. For Project Traffic and Intersection Analysis Reports for use in district planning and PD&E studies, the following analysis should be performed:
    1. Perform intersection analysis utilizing the most recent version of the HCS software. Adjust signal timing plans and lane configurations, as necessary, to obtain an acceptable LOS. Justification must be made for all lanes added above and beyond the existing conditions.
    2. Perform arterial analysis utilizing the most recent version microsimulation software Adjust intersection analysis, as necessary, to obtain an acceptable LOS.
  2. For ESAL forecasting to be used in pavement design, perform LOS analysis utilizing the appropriate LOS spreadsheet. The LOS “D” volume derived for the appropriate number of lanes can be utilized in calculating the 18-KIP ESAL.

Final Review and Documentation

  1. Perform final quality control (QC) review for consistency and reasonableness of projections. The assessment of reasonableness should examine traffic projections in comparison with observed traffic and historical trends, prospective roadway improvements, and land use projections. The QC review should also perform error checks to ensure that input numbers have been correctly transcribed and traffic forecasting computations have been done correctly.
  2. Prepare Project Traffic Forecasting Report documenting procedures, assumptions, and results.
  3. Prepare Project Traffic Forecasting Certification Statement and 18-KIP ESAL Forecasting Certification Statement (see Figure 4-6 and Figure 4-7). Refer to Project Traffic Forecasting Procedure, Topic No. 525-030-120, and obtain all authorized signatures.