Attachment 7

NYSDOT SEASONAL ADJUSTMENT MEMO 
NYSDOT Planning & Strategy Group March 28, 2003

Discussion with DEC Air Quality Staff on Traffic Seasonal Adjustment Factors

This documents the recommendations by NYSDOT Planning & Strategy Group’s Data Analysis &

Forecasting Section regarding seasonal adjustment factors for estimating mobile source emissions, per

our phone discussion of March 25, 2003.

To incorporate the full spectrum of Factor Groups (FG) developed for seasonally adjusting AADTbased DVMT estimates for summer and winter travel periods, it has been suggested that application of

the FGs developed for rural areas be used where appropriate. DOT recommendations are as follows:

• Use FG 30 in all large urban areas

• In counties with large urban areas, use FG 30 in urban areas and FG 40 in rural areas.

• In counties with only small urban areas where the county is essentially rural in nature,

use FG 40 in the small urban areas and FG 60 in the rural areas.

In applying the above general rule to all the counties in New York State, we also reviewed the

lane mileage factor group assignments for the State touring routes by the Department’s Regional

Offices, in order to determine which Factor Groups dominate the county. Background on the

Factor Groups is provided below. 

The attached spreadsheet uses the 1999 HPMS Grouped Functional Class DVMT and

Forecast to show our recommendation for applying these Factor Groups. A column was

added next to the urban area codes (UAC) called “SeasADJ” that provides FG codes for each

county’s urban and rural areas. 

The statewide weighted average FG equals 34.5, indicating the seasonality of total DVMT is closer to

that of urban areas than rural areas (FG 30 than FG 40 or 60). Since the weighted average is almost

midway between FG 30 and 40, however, this seems to support changing to the use of all FG groups

for improved estimates of seasonal traffic volume. 

It was noted that these are only recommendations by NYSDOT Planning & Strategy, and the ultimate

decision lies with DEC and their partners on the air quality interagency consultation group, including

NYSDOT’s Environmental Analysis Bureau. While we have kept EAB in the loop and let them know

of our recommendations, this does not necessarily represent endorsement of these recommendations by

EAB at this time.
NYSDOT Planning & Strategy Group March 28, 2003

Background on Seasonal Adjustment Factor Groups:

The DEC “Radian Report” for establishing 1990 base-year emissions estimates used the FG 30

seasonal factor to develop summer ozone mobile source emission estimates. The majority of DOT’s

continuous traffic counter stations for developing seasonal adjustment factors are located in urban

areas, so FG 30 is a more robust sample of traffic counts compared to FG 40 and 60. However, it

may be an improved procedure to use the seasonal adjustment factors for the rural areas as well. 

These factor groups are roughly constructed as follows:

FG 30 - This factor group is the least variable on a seasonal basis, as it is generally dominated by peak

period work trip travel flow. FG 30 originally had a total of 24 continuous traffic counter locations

statewide to develop these factors in 1990. Now a total of 70 are used to develop the FG 30 factors. 

FG 30 mostly consists of facilities classified as urban interstate, urban principal arterial and urban minor

arterial. 

FG 40 - This category is “moderately seasonal.” This group originally had a total of 17 stations, and

now has a total of 55 stations statewide. FG 40 consists mostly of locations classified as rural minor

arterial and rural major collectors.

FG 60 - This group shows the most “highly seasonal” variability in traffic volume throughout the year. 

The group originally had 8 continuous counter stations used to develop the factors, and now has 36. 

FG 60 mostly consists of rural interstate and rural principal arterial locations. 

The 1999 DVMT Inventory was based on the roadway functional classifications (FC) in place at the

time, which were developed consistent with the 1990 Census urban boundaries. These urban

boundaries will be adjusted based on the 2000 Census, which will lead to some changes in the

roadway functional classifications. We know from the new population figures that the urban areas in

New York have changed in some cases relative to the size of the rural areas. Therefore, any

calculation performed with the 1999 DVMT inventory may slightly overestimate DVMT traveling on

roadways with rural FCs and slightly underestimate travel on urban FCs.