Appendix C 

Proposed Method to Improve Temporal Distribution 

Of Gridded On-road Motor Vehicle Emissions 

Background

This document discusses a proposed method to improve the temporal resolution 

of gridded on-road motor vehicle emission inventories. The Integrated 

Transportation Network (ITN) and the Direct Travel Impact Model (DTIM) are 

currently used to develop the hourly, spatial distribution of on-road emissions. 

The ITN is a seamless on-road transportation network for California that contains 

about 250,000 roadway segments (or ‘links’). A revision to the ITN, called ITNv2, 

has recently been completed. The ITNv2, like the original ITN, is developed from 

the transportation networks of several regional transportation planning agencies 

(RTPAs) as well as the California Department of Transportation’s (Caltrans) 

statewide model. The hourly emission maps produced from the ITNv2 and DTIM 

are used as spatial surrogates to map the county emission estimates produced 

from ARB’s on-road motor vehicle emission model, EMFAC. 

Travel activity data used in the ITNv2 represent travel during the core weekdays, 

Tuesday, Wednesday and Thursday. We do not propose to make adjustments to 

the hourly emissions of light or medium duty vehicles produced by 

ITNv2/DTIM/EMFAC for these core days. However, (except for southern 

California) hourly distributions of heavy-duty vehicles are made by ratios of light 

duty travel. Therefore, we propose to use count data, which is distinguished by 

the vehicle type and number of axles, to temporally redistribute heavy-duty 

vehicle activity for all days, and light and medium duty vehicles for all non-core 

days. We place a special emphasis on weekend days since many pollutant 

episodes contain one or more weekend days. 

In previous modeling work for CCOS using the first version of the ITN, 

adjustment factors were applied to approximate travel on weekend days by 

lowering the volume on weekend days. This adjustment was based on a traffic 

study conducted by UC Davis for CCOS. See the description of the previous 

methodology at the end of this document. Heavy duty NOx emissions were 

further reduced to reflect lowered weekend truck travel. However, all vehicle 

types were assigned the same diurnal patterns. This is of concern because 

trucks are known to have different diurnal distributions than cars and they have 

high NOx emissions. 

Due to this concern, the Weekend Truck Subcommittee of the northern California 

SIP Gridded Inventory Coordination Group (GICG) was formed to investigate a 

way to improve day-of-week adjustments, for vehicle types as needed, but 

particularly for heavy-duty trucks. Participants in the subcommittee are members 
of the GICG with particular knowledge and/or interest in improving the 

adjustment factors and include representatives from Caltrans, ARB, Bay Area 

AQMD, San Joaquin Valley Unified APCD and Alpine Geophysics (the developer 

of the ITN). Since Caltrans count data are available to improve the temporal 

distribution of emissions by vehicle type, the UC Davis adjustments will not be 

used in the ITNv2. 

 

As a note of interest, while we are proposing ways of improving the temporal 

distribution of emissions, the spatial distribution is only altered at the county level. 

One way to correct the spatial distribution of on-road motor vehicle emissions on 

weekend days is through the development of a weekend travel demand model. 

Some states have begun to investigate the development of a weekend travel 

demand model; we are unaware of any such pursuit in California largely due to 

high cost of developing a weekend model. 

Description of Current Work

Leonard Seitz of Caltrans acquired Automatic Vehicle Classifier (AVC) count 

data from Joe Avis in Caltrans Traffic Ops from many (~139) sites in the state for 

calendar year 2004 (see Figure 1). Caltrans staff prepared hourly day of week 

factors for (1) passenger cars (LD), (2) light and medium duty trucks (LM), and 

(3) heavy-heavy duty trucks (HHDT). Caltrans count data are separated using 

the Federal Highway Administration (FHWA) vehicle classification scheme (see 

Table 1). Passenger cars are defined as FHWA classes 1 through 3. Light and 

medium heavy duty trucks are defined as FHWA classes 7 and 8. Heavy-heavy 

duty trucks are defined as FHWA classes 9 through 14. Separate factors were 

prepared for each Caltrans District. One or more counties may fall into a single 

District. All counties within each Caltrans district will receive the same 

adjustment. Figure 2 shows a map of county and Caltrans district. Only counts 

during the summer of 2004 were used, specifically the months of June, July and 

August excluding data from July 2-5 to remove unusual traffic patterns around 

the July 4th holiday. 

 

Proposed Method

Temporal on-road activity adjustments by county will be made for: 

1. Heavy duty vehicles – all days 

2. Light duty vehicles – Friday, Saturday, Sunday, Monday 

Daily total activity (daily VMT) adjustments will be made for all vehicle types for 

Friday, Saturday, Sunday, and Monday. Tuesday, Wednesday and Thursday are 

considered as one day. Adjustments applied to heavy-duty vehicles on 

Tuesdays, Wednesdays and Thursdays will be the same for each of the three 

days. 
Since it is EMFAC emission estimates that are being adjusted to derive the final 

on-road inventory, the relation between EMFAC vehicle classes and Caltrans’ 

adjustment factors is shown here. 

EMFAC Class Description Caltrans’ Factor 

 1 LDA LD 

 2 LDT1 LD 

 3 LDT2 LD 

 4 MDV LD 

 5 LHDT1 LM 

 6 LHDT2 LM 

 7 MHDT LM 

 8 HHDT HHDT 

 9 Other Bus No data in ITNv2 

 10 School Bus Unadjusted on weekdays, 

 zero on weekend days 

 11 Urban Bus LD 

 12 Motorhomes LD 

 13 Motorcycles LD 

where LD based on count data for FhwA classes 1 through 3 

 LM based on count data for FhwA classes 7 and 8 

 HHDT based on count data for FhwA classes 9 through 14 

Here are the proposed steps for the adjustments: 

1) Sum the hourly volumes by vehicle type and county on the ITNv2 network. 

2) For heavy-duty core days there will be an hourly redistribution but no daily 

VMT adjustment. Light duty vehicle emissions from EMFAC will not be 

adjusted at all for core days. 

3) For Friday, Saturday, Sunday, and Monday, develop a set of ratios of 

Caltrans daily VMT to core days. That is, for Saturday to Tues-Thurs (etc), 

by Caltrans 3 factor types LD, LM and HHDT. 

4) Apply Caltrans daily factors by county, and secondly, apply Caltrans’ new 

hourly distributions by county to ITNv2 link activity. 

5) Run DTIM with revised ITNv2 activity. 

 

6) Run EMFAC with day specific temperatures. 

7) Adjust DTIM output emissions to EMFAC weekday by county. 
8) For Friday, Saturday, Sunday, and Monday, apply daily ratios from step 3 

to hourly DTIM emissions by county. 

To summarize, for core days light- and medium-duty vehicle emissions will equal 

EMFAC emissions by county and hour. For core days, heavy-duty emissions will 

equal EMFAC but have Caltrans hourly distribution. 

For Friday through Monday, EMFAC weekday emissions will be scaled to reflect 

Caltrans day of week factors. 

Previous Method for Making Weekend Adjustments

EMFAC does not make weekend day estimates, so EMFAC daily emissions were 

scaled to represent weekend estimates based upon available traffic count data. 

The ITN version 1 network has activity estimates for a weekend day. In order to 

approximate an EMFAC weekend day inventory, ARB staff scaled each county’s 

EMFAC pollutants, except NOx, by the ratio of ITN VMT for a weekend day 

divided by the ITN VMT for a weekday for each county. The VMT for a weekend 

day was determined based on vehicle counts gathered and analyzed by Dr. Deb 

Neimeier of UC Davis. The county-specific ratio was applied to all vehicle classes 

and pollutants, except for NOx emissions from heavy-duty diesel trucks. Since 

heavy-duty diesel VMT is drastically reduced on weekends and NOx emissions 

from heavy-duty trucks are large, ARB staff multiplied the weekday NOx 

emissions by a factor of 0.536 (reduction of 46.4%) to estimate the NOx 

emissions on a weekend day. The factor of 0.536 represents the average 

reduction in NOx for a weekend day in the South Coast Air Basin. This NOx 

reduction was derived from Caltrans Weigh-in-Motion traffic counts that can 

differentiate between vehicle types by weight. 
Figure 1 Caltrans Weigh-In-Motion Data Sites 
Table 1 Federal Highway Administration (FHwA) Vehicle Classification 

Graphic Depiction FHWA 

Class 

Description 

1 Motorcycles 

2 Passenger Cars (With 1- or 

2-Axle Trailers) 

3 2 Axles, 4-Tire Single 

Units, Pickup or Van (With 

1- or 2-Axle Trailers) 

4 Buses 

5 2D - 2 Axles, 6-Tire Single 

Units (Includes 

Handicappe-Equipped Bus 

and Mini School Bus) 

6 3 Axles, Single Unit 

7 4 or More Axles, Single 

Unit 

8 3 to 4 Axles, Single Trailer
Graphic Depiction FHWA 

Class 

Description 

9 5 Axles, Single Trailer 

10 6 or More Axles, Single 

Trailer 

11 5 or Less Axles, MultiTrailers 

12 6 Axles, Multi-Trailers 

13 7 Axles, Multi-Trailers 

No graphic available 14 5 Axles: 3 axle tractor 

pulling a 2 axle trailer 

(FHWA considers this type 

of truck a class 9;Caltrans 

counts these trucks 

separately for operational 

purposes.) 
Figure 2 Caltrans District and County Boundaries