Collection and Estimation of Annual Average Daily Traffic on NonFederal Aid System Roads 

VIRGINIA DEPARTMENT OF TRANSPORTATION

INNOVATIVE PROCEDURES IN TRAFFIC VOLUME 

ESTIMATION 

CASE STUDY

FHWA-SA-17-036

Federal Highway Administration Office of Safety 

Roadway Safety Data Program 

http://safety.fhwa.dot.gov/rsdp/

February 10, 2016
VIRGINIA DEPARTMENT OF TRANSPORTATION CASE STUDY 

1

Notice 

This document is disseminated under the sponsorship of the U.S. Department of 

Transportation in the interest of information exchange. The U.S. Government assumes no 

liability for the use of the information contained in this document. 

The U.S. Government does not endorse products or manufacturers. Trademarks or 

manufacturers’ names appear in this report only because they are considered essential to the 

objective of the document. 

Quality Assurance Statement 

The Federal Highway Administration (FHWA) provides high-quality information to serve 

Government, industry, and the public in a manner that promotes public understanding. 

Standards and policies are used to ensure and maximize the quality, objectivity, utility, and 

integrity of its information. FHWA periodically reviews quality issues and adjusts its programs 

and processes to ensure continuous quality improvement. 
VIRGINIA DEPARTMENT OF TRANSPORTATION CASE STUDY 

2

TECHNICAL DOCUMENTATION PAGE 

1. Report No.

FHWA-SA-17-036

2. Government Accession 

No.

3. Recipient's Catalog No.

4. Title and Subtitle

Innovative Procedures in Traffic Volume Estimation

5. Report Date

February 2017

6. Performing Organization Code

7. Author(s) 

Ioannis Tsapakis, William Holik, Anita Vandervalk, Shawn Turner, and John 

Habermann

8. Performing Organization Report No.

9. Performing Organization Name and Address

Texas A&M Transportation Institute (TTI)

Texas A&M University System 

3135 TAMU

College Station, TX 77843-3135

10. Work Unit No.

11. Contract or Grant No.

DTFH6116D00004

12. Sponsoring Agency Name and Address

Federal Highway Administration Office of Safety 

1200 New Jersey Ave., SE

Washington, DC 20590

13. Type of Report and Period

Case Study, August 2016–February 

2018 

14. Sponsoring Agency Code

FHWA

15. Supplementary Notes

The contract manager for this report was Stuart Thompson, Office of Safety. 

16. Abstract

This case study presents how the Virginia Department of Transportation (VDOT) estimates traffic volumes on secondary 

local roadways using a trip generation method instead of taking short-duration counts in the field. The use of trip 

generation estimates was an outcome of a review process that aimed to reduce data collection costs and achieve manpower 

savings by establishing a local secondary count program. As part of this program, VDOT provided guidelines to its staff on 

how to identify eligible traffic links and generate traffic volume estimates using the trip generation method. According to 

this program, region and district staff are responsible for reviewing candidate segments using aerial photos, determining 

potential development and roadway connectivity, evaluating roadway eligibility criteria, documenting the trip generation 

process used, developing estimates, and submitting the estimates to the Central Office–Traffic Engineering Division. After 

implementing the method, VDOT realized several benefits, most of which are associated with time and cost savings.

Because of the positive experience and feedback received with this effort, VDOT will continue to promote and improve the 

use of the trip generation method.

17. Key Words: 

AADT, data collection, traffic volume estimation, 

trip generation estimates, traffic counts

18. Distribution Statement

No restrictions.

19. Security Classif. (of this report)

Unclassified

20. Security Classif. (of this 

page) Unclassified

21. No. of Pages

20

22. Price

Form DOT F 1700.7 (8-72) Reproduction of Completed Pages authorized
3

VIRGINIA DOT CASE STUDY 

ACRONYMS 

Table 1. Acronyms. 

Acronym Description

AADT Annual Average Daily Traffic

ADT Average Daily Traffic

CCS Continuous Count Station

CO-TED Central Office–Traffic Engineering Division

FHWA Federal Highway Administration

ITE Institute of Transportation Engineers

VDOT Virginia Department of Transportation
VIRGINIA DOT CASE STUDY 

4

EXECUTIVE SUMMARY 

This case study presents how the Virginia Department of Transportation (VDOT) estimates 

traffic volumes on secondary local roadways using a trip generation method instead of taking 

short-duration counts in the field. The use of trip generation estimates was an outcome of a 

review process that aimed to reduce data collection costs and achieve labor savings by 

establishing a local secondary count program. As part of this program, VDOT provided 

guidelines to its staff on how to identify eligible traffic links and generate traffic volume 

estimates using the trip generation method. According to this program, region and district staff 

are responsible for reviewing candidate segments using aerial photos, determining potential 

development and roadway connectivity, evaluating roadway eligibility criteria, documenting the 

trip generation estimate process used, developing estimates, and submitting the estimates to 

the Central Office–Traffic Engineering Division. After implementing the method, VDOT realized 

several benefits, such as reduction in data collection costs, time savings, increased efficiency of 

VDOT’s data collection program, elimination of duplicate data collection efforts, easy validation 

of existing traffic volume estimates, and quick access and dissemination of traffic estimates 

among various entities within VDOT. Because of the positive experience and feedback received 

with this effort, VDOT will continue to promote and improve the use of the trip generation 

method.
VIRGINIA DOT CASE STUDY 

5

TABLE OF CONTENTS 

INTRODUCTION ....................................................................................... 7

BACKGROUND .......................................................................................... 8

TRIP GENERATION ESTIMATES ............................................................ 9

IMPLEMENTATION PROCESS ................................................................................ 9

SELECTION OF TRAFFIC LINKS ..........................................................................10

TRAFFIC VOLUME ESTIMATION .........................................................................11

COST AND OTHER CONSIDERATIONS ............................................................14

LESSONS LEARNED ................................................................................15

REFERENCES ............................................................................................17
VIRGINIA DOT CASE STUDY 

6

LIST OF TABLES 

Table 1. Acronyms. ............................................................................................................ 3

LIST OF FIGURES 

Figure 1. Examples: (a) Step 1; (b) Step 2. ...................................................................12

Figure 2. Examples: (a) Fariba Drive; (b) Pine Knot Drive. ........................................13
VIRGINIA DOT CASE STUDY 

7

INTRODUCTION 

The purpose of this case study is to highlight current practices at the Virginia Department of 

Transportation (VDOT) regarding traffic volume estimation using trip generation techniques. 

On September 23, 2009, VDOT’s regional traffic engineers agreed that a trip generation 

estimate method could be used for certain secondary local roadways rather than actual field 

counts. The use of trip generation estimates was an outcome of a review process that aimed to 

reduce data collection costs and achieve work force savings by establishing a local secondary 

count program. As part of the local secondary program, VDOT established criteria for eligible 

traffic links and provided guidance to its staff on how to use the trip generation theory based 

method. The Institute of Transportation Engineers (ITE) endorsed and published this method. 

Over time, VDOT has realized several benefits from using the trip generation method to

estimate traffic volumes. Most benefits involve time, cost savings, and include: 

 Reduced data collection costs because using the trip generation method requires fewer 

resources and fewer person-hours than conducting short-duration counts in the field. 

 Minimized need for purchasing, maintaining, and calibrating traffic equipment, resulting in 

additional cost savings. 

 The trip generation method is significantly faster than collecting traffic data in the field 

and processing the data in the office. VDOT staff can allocate the time saved to other 

tasks. 

 VDOT’s data collection program increased efficiency by eliminating potential duplicate 

data collection efforts. 

 Easy validation of existing traffic volume estimates. 

 Quick access of traffic volume estimates by entities within VDOT. 

 Fast and easy generation of traffic volume estimates even in cases where the installation 

of portable traffic equipment and collection of short-duration data is difficult (e.g., during 

inclement weather conditions or when pavement is covered with snow or ice). 
VIRGINIA DOT CASE STUDY 

8

BACKGROUND 

VDOT operates the third largest state-maintained highway system in the country, including 

secondary roads. They have established an extensive traffic-monitoring program. Specifically, 

VDOT maintains 58,430 centerline miles of roadway, which is comprised of approximately 78 

percent of the 74,748-centerline miles of public roads. VDOT has collected roadway inventory 

data for 85,824 (95 percent) of the 90,614 intersections in the Commonwealth. 

VDOT’s traffic monitoring program includes more than 600 continuous count sites (CCSs) on 

all roadway functional classes. VDOT has counted approximately 98 percent of Virginia’s local 

roads. District office personnel, who are familiar with local roads in their jurisdiction, take 

local-road traffic counts on a six-year cycle. From 2006 through 2015, VDOT collected 125,202 

24-hour counts on roadway segments functionally classified as local. Unlike many agencies, 

VDOT does not use adjustment factors to expand the short-duration counts to annual average 

daily traffic (AADT) estimates. Instead, it uses the raw average daily traffic (ADT) value of each 

count. 
VIRGINIA DOT CASE STUDY 

9

TRIP GENERATION ESTIMATES 

IMPLEMENTATION PROCESS 

According to VDOT’s secondary program, region and district staff performs several actions 

that mainly involve: 

 Reviewing candidate segments using aerial photos, 

 Determining potential development and roadway connectivity, 

 Evaluating whether specific criteria are met, 

 Documenting the trip generation estimate process used, and 

 Developing and submitting the estimates to the Central Office–Traffic Engineering 

Division (CO-TED). 

This document describes these actions and provides relevant estimation examples for those 

interested in learning how to use the methods. 

Implementation Guidelines 

To support the implementation of the trip generation method, VDOT developed the following 

guidelines: 

 Roadways that are subject to use of trip generation techniques for traffic volume 

estimates are those that have one point of entry (e.g., cul-de-sacs), are residential in 

nature, and have a length of 0.5 mile or less. 

 Region/district staff should choose which method to use (24-hour traffic count volume 

or trip generation estimate). 

 Regional traffic engineers should coordinate their decision with their VDOT residency 

customers. 

 Traffic volume estimates should represent the average volume over the entire roadway 

length. This value will typically be less than the highest traffic volume (e.g., that found at 

the roadway entrance). 

 Regions should provide the estimate using the same county listing submission method 

currently in use to report traffic count results. Traffic volume estimates based on trip 
VIRGINIA DOT CASE STUDY 

10

generation theory should have the date the estimate was developed and the letter “M”

placed next to the value indicating whether it is a “manual” estimate. 

 The Central Office records the technique creation date and estimates in a database as 

manual estimates, and publishes them with an identifier indicating the same. 

 In future count years, the county listings provided to the regions to support traffic count 

collection and local secondary roadway volume reporting to the Central Office will 

include information that identifies the past estimate as manual, along with the date of the 

estimate. Regions decide the count cycles for updating manual estimates. 

SELECTION OF TRAFFIC LINKS 

VDOT developed criteria and guidance to evaluate sites for the trip generation method. 

District staff reviews each candidate site using recent aerial images to determine development 

and roadway connectivity. If VDOT staff can make a determination using aerial photos, VDOT 

recommends using the trip generation method only for traffic links that meet the following 

criteria: 

 Land use is limited to single-family detached housing (ITE Trip Generation Code 210). 

For other land uses, it may not be possible to determine the number of dwelling units 

(or other independent variables) from an aerial photo. In the case of higher-density 

residential, this limitation also eliminates the need to judge which trip generation code 

to use, which may also be difficult to determine from an aerial photo. The majority of 

cul-de-sacs in the Northern Region consist of single-family homes. 

 No alternative means of motor vehicle access is available, even by private streets or 

driveways. 

 Unusual land uses are nearby that might cause traffic volumes to differ significantly from 

the trip generation rates. For example, a Metrorail station may induce more bicycle and 

pedestrian trips and fewer vehicle trips, or it may induce more vehicular trips if it 

attracts on-street parking. 

 Occasionally, a private street diverges from a VDOT-maintained cul-de-sac. 

Consideration of these cul-de-sacs for the trip generation method is dependent on 

meeting all the criteria above, even though the private streets may lead to additional 

development, as long as the combined length of the public and private streets are less 

than the 0.5-mile threshold. 

District staff retains the flexibility to conduct an actual traffic count on any link that exhibits 

unusual traffic conditions that do not meet the criteria defined above. 
VIRGINIA DOT CASE STUDY 

11

TRAFFIC VOLUME ESTIMATION 

VDOT provides its staff with a description of the trip generation method along with estimation 

examples. According to VDOT, a simple method of computing traffic volumes for cul-de-sacs is 

to multiply the total number of households on the cul-de-sac by the trip generation rate. 

However, this would approximate the volume at the point where the cul-de-sac intersects the 

connecting roadway. The volume on a cul-de-sac is highest at this point since all trips from the 

cul-de-sac must pass it. The volume becomes lower toward the bulb because households along 

the stem typically travel away from the bulb. To capture this trend, the trip generation method 

estimates the average volume per weekday along the cul-de-sac as follows: 

ν =

∑ (gn×dn)

H

n=1

L

 (1) 

Where: 

ν = number of trips per weekday using a cul-de-sac, averaged according to the length of 

 the cul-de-sac. 

H = households with driveways on the cul-de-sac. 

gn = number of trips per weekday generated by the nth household. 

dn = portion of the length of each trip to and from the nth household that uses the cul-de- 

 sac, taken as the distance along the cul-de-sac between the nth household’s driveway 

 and the end of the stem. 

L = length of the cul-de-sac. 

Because VDOT uses this process for only one land use type, it considers the same g value for 

every household. VDOT extracts this value from the ITE Trip Generation Manual. For singlefamily detached housing (Code 210), g = 9.57 trips per weekday per dwelling unit, according to 

the 8th edition of the manual. VDOT rounds this rate to 10 trips per weekday for 

computational ease. 

VDOT makes a further distinction between “bulb households,” with driveways in the bulb of 

the cul-de-sac, and “stem households,” with driveways in the stem. Trips to and from bulb 

households typically use the cul-de-sac for approximately its entire length, particularly when 

considering additional maneuvering in the bulb. Stem household trips typically use the cul-de-sac 

only from their driveways to the connecting road. Equation (1) simplified as follows: 

ν = g × (Hb + Hs ×

d

L

) (2) 
12

VIRGINIA DOT CASE STUDY 

Where: 

g = 10 trips per weekday per household. 

Hb = number of bulb households. 

Hs = number of stem households. 

The ratio d/L is a dimensionless constant since both d and L are expressed in units of length. It 

represents the average fraction of the cul-de-sac used by trips to and from stem households. 

Stem household driveways are assumed to be distributed uniformly along the length of the culde-sac, so d/L = 0.5. VDOT further simplifies Equation (2) as follows: 

ν = 10 × (Hb + 0.5Hs

) (3) 

Although it is possible to compute a traffic 

volume estimate without differentiating between 

bulb and stem households, VDOT does not do 

so because it wants to produce estimates that 

are more accurate. Most cul-de-sacs do not 

have driveways distributed uniformly along their 

length. Site plans often distribute driveways 

more densely in the bulb of the cul-de-sac than 

along the stem. Likewise, connecting roadways 

often serve households at the corner of a culde-sac resulting in fewer driveways. 

VDOT established the following five steps for 

estimating traffic volumes for eligible traffic 

links. 

Step 1: Draw an imaginary line at the entrance 

of the cul-de-sac, where it joins the connecting 

road, and count the number of houses whose 

primary driveway can only be reached by 

traveling across the line. For example, on the 

traffic link shown in Figure 1a, there are nine 

houses.

Step 2: Draw another imaginary line between 

the bulb and the stem of the cul-de-sac and 

count the number of houses whose primary 

Figure 1. Examples: (a) Step 1; (b) Step 2.

(a) 

(b) 

Source: Process for implementing the trip generation 

method as an alternative to secondary local counts, VDOT 

Northern Region (Rev. 12/2009).
13

VIRGINIA DOT CASE STUDY 

driveway can only be reached by traveling across this line. For example, on the cul-de-sac 

shown in Figure 1b, there are three houses. 

Step 3: Add the results of Steps 1 and 2, and multiply the sum by 5. The calculation in this step 

is based on Equation (3). For example, from the first two figures, the result is (9 + 3) x 5 = 60. 

Step 4: Report the result of Step 3 as the number of trips per day on a spreadsheet provided 

by the Central Office. Use the letter “M” to indicate that the value resulted from a “manual”

estimate rather than a physical count. Also, report the date the estimate was prepared. 

Step 5: Periodically send the spreadsheet back to CO-TED to import it into the traffic 

monitoring system. This occurs at least once per year. The results are reported in annual 

publications. 

VDOT provides its staff with additional aerial photos (Figure 2) and estimation examples to 

highlight differences between links that have different roadway and geometric characteristics. 

Private Streets

Figure 2. Examples: (a) Fariba Drive; (b) Pine Knot Drive.

Source: Process for implementing the trip generation 

method as an alternative to secondary local counts, 

VDOT Northern Region (Rev. 12/2009).
14

VIRGINIA DOT CASE STUDY 

The first three steps of the traffic volume estimation process result in the following: 

Figure 2a: 

Step 1: 17 houses. 

Step 2: 4 houses. 

Step 3: (17 + 4) x 5 = 105 trips per day. 

Figure 2b: 

Step 1: 38 houses. 

Step 2: 17 houses. 

Step 3: (38 + 17) x 5 = 275 trips per day.

Figure 2a shows a simple case of a traffic link that consists of a bulb and a stem. In Figure 2b, 

the cul-de-sac has two private-street connections through the bulb, taken into consideration in 

Step 2, and two connections along the stem. Distribution of the connections along the stem is 

relatively uniform along the cul-de-sac’s length, so the trip generation computation is valid. 

COST AND OTHER CONSIDERATIONS 

In general, the cost to generate traffic volume estimates is significantly lower than collecting 

field data. Some Districts apply the trip generation method in-house, while others hire a 

temporary service contractor. As of February 2016, a typical hourly rate of these contractors 

ranged between $20 and $30 per hour. Depending on the complexity and the number of the 

houses along a roadway link, VDOT staff or its contractors spend between 15-30 minutes to 

generate a traffic volume estimate for one link. 

Most contractors are responsible for both conducting counts in the field using portable traffic 

equipment and generating traffic volume estimates using the trip generation method. Some 

Districts ask their contractor(s) not to rely heavily on aerial photos, but visit the roadway links 

of interest and collect necessary information and data in the field. To save time and reduce 

travel costs, contractors try to do both (i.e., conduct short-duration counts and visit eligible 

links that are close to the counts) in one trip. Other Districts allow their contractor(s) to apply 

the trip generation method at the office and ask them to visit the field in case the aerial photos 

are old, trees cover driveways and houses, and there is significant construction activity in the 

examined area. 
15

VIRGINIA DOT CASE STUDY 

LESSONS LEARNED 

VDOT is innovative, creative, and proactive when it comes to implementing new strategies. 

This practice is part of VDOT’s culture and accompanies its strategy of incorporating user 

needs and ideas into the development of new business practices and methods. VDOT’s lessons 

learned from using trip generation estimates can be beneficial to other agencies. The most 

important lessons learned pertaining to this practice are summarized below. 

 The use of trip generation estimates has minimized the need to carry out traffic counts 

on qualified links by yielding time and cost savings for the department. 

 It is cheaper to use state resources and labor than to hire contractors in most cases. 

 Keys to successful application of the trip generation method include a clear 

implementation plan and specific assignment of roles and responsibilities to different 

parties within the agency. 

 Developing requirements and guidelines can be a time-consuming process, but the 

benefits realized outweigh the development effort. 

 Developing systematic instructions and providing illustrated examples facilitated the 

implementation process and helped VDOT staff easily understand how to apply the trip 

generation method. 

 Providing information regarding the trip generation theory and explaining how formulas 

can be simplified can help region and district staff better understand the basic principles 

of this method and make appropriate assumptions. 

 Having people with a strong background in data collection and traffic volume estimation 

helps with development of the guidelines. 

 Agencies should inform managers and executives on cost savings and other benefits 

achieved through adopting more-efficient procedures. 

 Staff must be willing to change existing business practices and learn how to use new 

methods. 

 Having the right people in the room helps with decision-making. 

 Agencies should consider the needs and preferences of all region and district staff. This 

helps secure buy-in and support across the board during and after the implementation 

phase. 
16

VIRGINIA DOT CASE STUDY 

 Agencies should estimate the cost for generating traffic volume estimates, compare it to 

the cost for conducting and processing short-duration counts, and relate it to the 

hidden cost of not having data on secondary local roads. 

 If aerial photos are old, the quality of the photos is low, trees cover houses and 

driveways or there is significant construction activity in the examined area, it is better to 

visit the examined link(s) and gather necessary data and information in the field than 

from aerial images. 

Because of the positive experience and feedback received with this effort, VDOT will continue 

to promote the use of the trip generation method.
17

VIRGINIA DOT CASE STUDY 

REFERENCES 

Phone interviews with: 

 John Bechtold, (Assistant Resident Engineer, Bristol District, VDOT) in discussion with 

the author. September 30, 2016. 

 Dan Dunnavant, (Traffic Data Analyst, Traffic Engineering Division, VDOT), in discussion 

with the author. September 30, 2016. 

 Ali Farhangi, (Assistant District Traffic Engineer, Culpeper District, VDOT), in 

discussion with the author. September 30, 2016. 

 Ralph Jones, (Traffic Engineering Division, VDOT), in discussion with the author. 

September 30, 2016. 

 Donald Logan, (Assistant District Traffic Engineer, Staunton District, VDOT), in 

discussion with the author. September 30, 2016. 

 Brett Randolph, (Assistant District Traffic Engineer, Salem District, VDOT), in 

discussion with the author. September 30, 2016. 

Federal Highway Administration (2016). Traffic Monitoring Guide, Office of Highway Policy 

Information, Washington, D.C. 

Institute of Transportation Engineers. (2008). Trip Generation Manual, 8th Edition, User’s Guide, 

ITE, Washington, D.C. 

Virginia Department of Transportation Northern Region. (2009). Process for Implementing the 

Trip Generation Method as an Alternative to Secondary Local Counts, Rev. 12/2009, VDOT, 

Richmond, VA. 

Virginia Department of Transportation (2015). Use of Trip Generation for Traffic Volume 

Estimation, VDOT, Richmond, VA. 

FHWA, Office of Safety 

Stuart Thompson, P.E. 

Federal Highway Administration 

202-366-8090 

Stuart.Thompson@dot.gov