The AADT, K, D, and T for the current year are available in the RCI database under Feature 331 (Traffic Flow Breaks). The information is updated annually, and the most current version of the traffic information should always be used for Project Traffic Forecasting.

To estimate AADTs along state roadways where no Continuous or Short-term sites are available, or roadways are not on the state system, a field data collection of short-term traffic counts must be conducted using either portable axle counters or portable vehicle counters. Appropriate SF and/or ACF should be applied to adjust the short-term ADT to obtain AADT using Equation 2-3.

Seasonal Factor Category Report

Seasonal Factors (SF) are provided in the Peak Season Factor Category Reports that can be generated and downloaded from Florida Traffic Online. Seasonal Factor Categories are groupings of continuous stations whose data are used to develop the seasonal factors. Seasonal Factor Categories are designed to be county specific with at least one “Countywide” Seasonal Factor Category for each County and one Seasonal Factor Category for each Interstate Road within each County. Additional Seasonal Factor Categories are developed to handle geographic differences within a single county (e.g., beach traffic vs. urban traffic). Seasonal Categories are represented by a four-digit number, where the first two (2) digits correspond to the county codes, and the second two (2) digits are a sequence number or an interstate number. It contains a verbal description of its intended use, and a maximum of eight (8)continuous count station numbers. Details of Seasonal Factor Categories are included in the Volume Factor Category Summary Report.

Figure 2-7 shows the volume factor categories for Hillsborough County (County Code 10). There are four (4) seasonal factor categories for Hillsborough County, one for Hillsborough Countywide represented by Code 1000, and the other three for the three interstate highways in Hillsborough County, I-4, I-275, and I-75, represented by 1004, 1027, and 1075, respectively. Only Category 1000 and Category 1004 reports are shown in Figure 2-7.

The weekly seasonal adjustment factors are presented in the Peak Season Factor Category Reports by category and by week. There can be 52 to 54 weekly factors, depending upon which day of the week January 1st falls.

If the Seasonal Factor (SF) is greater than one (1), then the raw counts were collected during low traffic volumes and must be adjusted upward to reach the annual average.

If the SF is less than one (1), then the raw counts were collected during high traffic volumes and must be adjusted downward to reach the annual average. Figure 2-8 shows the Peak Season Factor Category Report for Hillsborough Countywide Category 1000.

Figure 2-7 Volume Factor Categories for Hillsborough County
Figure 2-8 Peak Season Factor Category Report for Hillsborough Countywide

Axle Factor Category Report

Axle Factor Categories are groupings of vehicle classification stations whose data is used to develop the Axle Correction Factors. The Axle Factor Categories are county specific, denoted by four-digit numbers with the first two (2) digits representing the county codes, and the second two (2) digits a sequence number. The Axle Factor Category Reports are facility specific and contain one or more groups. Care should be taken when selecting the Axle Correction Factors to ensure factors from correct categories are used. Figure 2-9 shows an example of Axle Correction Factors for Hillsborough County for 2022.

The Axle Correction Factors are also reported by category and by week. There could be 52 to 54 weekly factors depending upon which day of the week January 1st falls. All axle adjustment factors are less than or equal to one (1). The axle adjustment factors are multiplied by the raw counts to lower axle counts into vehicle count estimates.

Figure 2-9 Axle Correction Factors for Hillsborough County

Example of Estimating AADT from Short-Term Traffic Counts

The following example shows the steps to be performed to estimate AADT from short-term traffic counts conducted along a highway section. In this example, three-day 72-hour traffic counts were taken by portable axle counters on Kenner Highway, approximately 550 feet north of Central Parkway from Tuesday, March 21, 2023 to Thursday, March 23, 2023 in Martin County.

IMPORTANT NOTE:
Short-term traffic counts should be reviewed for reasonableness and consistency before applying adjustment factors to estimate the AADT. Follow the tips below to check the short-term counts:
  • Check consistency in daily counts if counts are taken for multiple days. Discard the bad daily counts. Recount if necessary.
  • Check differences between daily counts and historical counts from Florida Traffic Online if available.
  • Check daily counts against turning movement counts (TMC) during the same hour, or 15-minute intervals if the TMCs are taken on the same day.
  • Check differences between directional hourly volumes and departure/approach volumes from the turning movement counts at adjacent intersections.
Step1:
Review Traffic Counts for Consistency and Reasonableness
Figure 2-10 shows the three-day short-term traffic counts collected on Kenner Highway. The directional counts and the total daily counts collected for the three weekdays are consistent. Hourly volumes for the three days also show a similar pattern. Therefore, traffic counts from all three days will be used to calculate the ADT.
$$ADT=\frac{37,915+37,987+38,023}{3}=37,975$$
Step2:
Assign a Seasonal Factor from the Peak Season Factor Category Report
There are four volume factor categories for Palm Beach County, three for the different geographic areas of the county, and one for I-95:
  • Category: 8900 EAST- A1A TO US 1
  • Category: 8901 CEN.-W OF US 1 TO I-95
  • Category: 8927 WEST-W OF I-95
  • Category: 8995 MARTIN I-95
The short-term traffic counts were collected in central Martin County between west of US 1 and I-95, an area covered by Category 8901. Therefore, the seasonal factor from Category 8901 corresponding to the week of March 20, 2022 - March 26, 2022 was assigned to this location and the value of SF is 0.95. (See Figure 2-11.)
Figure 2-10 Sample Short-Term Traffic Counts
Figure 2-11 2022 Peak Season Factor Category Report for Category 8901
Step3:
Assign an Axle Correction Factor (ACF) from the Weekly Axle Correction Factor Category Report
Similar to Seasonal Factors, the ACF is obtained from the Weekly Axle Correction Factor Category Report. The ACFs are reported by facility, segment, and week. For roadways that do not belong to any of the included facility categories, the ACF for countywide rural, countywide urban, or countywide category can be used. There are 17 ACF categories for Martin County. The category that is most suitable for Kenner Highway is Category 8909 - SR 76, I-95 - SR A1A. The ACF for Category 8909 corresponding to the week of March 20, 2023 - March 26, 2022 is 0.98. (See Figure 2-12.)
Figure 2-12 Weekly Axle Factory Category Report for Category 8909
Step4:
Estimating AADT by Applying Adjustment Factors
$$AADT=ADT\times{SF}\times{ACF}$$ $$AADT=37,975\times{0.95}\times{0.98}=35,345$$ $$AADT=35,500 (After\space{applying}\space{Rounding})$$