Summary of Tract-to-Tract Commuter Flows by Type of Geographic Area. A useful way of comparing the general pattern of tract-to-tract commuter flows across multiple data sets is to summarize them into categories. For this analysis, tract-to-tract worker flow data from the LED and the census are compared by summarizing them into categories of a "flow typology" based on a simple classification scheme of the type of geographic area in which the tracts are located. Tracts located in the central city of a metropolitan statistical area were classified as "urban." Tracts located outside metropolitan central cities but still within the urbanized area were classified as "suburban." Tracts located within urbanized areas that are outside any metropolitan area were classified as “non-metro urbanized”. Finally, tracts located in the non-urbanized remainder of metropolitan areas or in non-metropolitan territory and not within an urbanized are were classified as “rural”. The resulting flow typology comprised intra-urban flows, intra-suburban flows, intra-rural flows, and flows between urban, suburban and rural areas. Flows to, from, and within tracts located in non-metropolitan urbanized areas also were differentiated in the same manner.

Origin-Destination Worker Flows at the Census Tract Level

Origins

Destinations

Total ProductionsUrban Suburban

Non-Metro Urbanized Rural

Percent of TripsCensusUrban 23.4 7.7 -- 1.8 32.8

Suburban 10.1 28.4 -- 3.4 41.9

Non-Metro Urbanized 0.1 -- 1.1 0.9 2.1

Rural 4.9 4.0 1.8 12.5 23.2

Total Attractions 38.3 40.1 2.9 18.7 100.0

LEDUrban 21.6 8.7 0.1 2.0 32.4

Suburban 12.1 28.2 0.1 3.5 43.9

Non-Metro Urbanized 0.2 0.1 1.1 1.0 2.5

Rural 4.8 4.5 1.7 10.2 21.3

Total Attractions 38.7 41.6 3.0 16.8 100.0

Note: In the Census data, 3,471 interchanges with a destination tract code of 999999, accounting for 200,280 trips, are not included in the table. These include 2,810 trips with urban origins, 1,876 trips with suburban origins, 23,138 trips with non-metro urbanized origins, and 174,456 trips with rural origins.-- Less than 0.5, so rounds to zero percent.Sources: 2001 LED O-D. Census 2000 Transportation Planning Package, Illinois, part 3.

Origins

Destinations

AverageUrban SuburbanNon-Metro Urbanized Rural

Census

Urban

21 12 8

17 18

Suburban

14 19 9

19 17

Non-Metro Urbanized

12 9 109

32 41

Rural

20 15 38

37 26

Average

19 17 48

29 19

Urban

5 3 2

3 4

Suburban

4 6 1

4 5

Non-Metro Urbanized

3 2 38

8 9

Rural

5 4 11

11 7

Average

5 4 10

7 5

LED

Average Number of Trips Per Tract-to-Tract Interchange, by Type of Flow

Tract-to-Tract Work Trips, by Type of Flow

Internal Work Trips Within the Same Tract by Area Type

Note: The LED data contained 124 tracts with no internal trips. The Census data contained 102 tracts with no internal trips. n/a = not applicableSources: 2001 LED O-D File. Census 2000 Transportation Planning Package, Illinois, part 3.

Distance DecayTransportation planners have shown that the number of work trips between places is

inversely related to distance. That is, all other things being equal, people tend to live close to where they work. So, for any destination tract, we would expect more trips from closer tracts and fewer trips from tracts farther away. A general form of this relationship is:

Tw = T0ebd-cd2

Where: Tw = Total work flows T0 = Internal trips d = Distance b,c parameters to be

estimated

When c = 0, the number of trips decreases exponentially with increasing distance. When b = 0, flows decline exponentially with the square of the distance.

For each of the four geographic area types, the ten destination tracts that attracted the most work trips in each dataset were analyzed (or 40 total destination tracts from each data set). Great circle distances were calculated between the geometric centers of each origin-destination pair. Flow between tracts greater than 150 miles apart were excluded from the analysis as anomalies.

Area Type

LED Census

Model R2 Model R2

Urbanln(Tw) = 10.6 - 0.037d 0.74 ln(Tw) = 11.3 - 0.072d 0.84

ln(Tw) = 12.3 - 0.11d + 0.0005d2 0.92 ln(Tw) = 13.2 - 0.15d + 0.0005d2 0.92

Suburbanln(Tw) = 9.9 - 0.039d 0.73 ln(Tw) = 10.5 - 0.070d 0.86

ln(Tw) = 11.7 - 0.12d + 0.0005d2 0.94 ln(Tw) = 12.4 - 0.15d + 0.0005d2 0.94

Non-Metro Urbanized

ln(Tw) = 7.9 - 0.028d 0.73 ln(Tw) = 8.5 - 0.058d 0.84

ln(Tw) = 9.2 - 0.08d + 0.0003d2 0.90 ln(Tw) = 10.4 - 0.14d + 0.0005d2 0.95

Ruralln(Tw) = 9.3 - 0.037d 0.75 ln(Tw) = 9.8 - 0.077d 0.83

ln(Tw) = 11.1 - 0.11d + 0.0005d2 0.96 ln(Tw) = 12.1 - 0.16d + 0.0006d2 0.95

Regression Results – transformed model (Ln Tw = ln T0 +bd – cd2)

ConclusionsThis exploratory analysis found that the total counts of workers in the LED data for the State of Illinois and selected Illinois counties, aggregated up from block records on the H-B File, compare favorably with the census and the ACS. However, the LED O-D File contains more than three times the number of tract-to-tract interchanges and nearly 1 million fewer work trips than the census. This results in a much lower average number of trips per interchange in the LED data compared with the census. But when the tract-to-tract interchanges and worker trips are summarized by type of origin-destination interchange or flow (e.g. suburban-to-urban or rural-to-suburban), the proportion of interchanges and trips by type of flow in the LED data is virtually identical to the census.

The results of this analysis, particularly the comparability of worker flow patterns between the LED and census data, demonstrate the potential utility of journey-to-work data from the LED Program for use in transportation planning. Because it is national in scope, the LED Program offers many advantages over the piecemeal approach of individual states trying to work with their state employment security agencies to obtain and use the employment data. Many states do not allow access to ES-202 data for research, so the LED Program is a means of accessing the data. The LED methodology is consistent for every state. The file that the LED Program uses to assign residence addresses to workers is consistent for every state and contains extensive confidential information not available outside the Census Bureau. Data from the LED Program are updated quarterly. The data are cost effective, costing much less than a survey or a census special tabulation.

The transportation data community has embarked on a coordinated effort to study and adjust to the ACS. It is an opportune time to thoroughly evaluate the utility of the LED data as well. Small-area data from the ACS will be based on multi-year averages. If the accuracy of the worker flow data from the LED Program is improved, the data could be used to evaluate and validate the origin-destination data from the ACS. Annual LED data could be used to measure changes in key worker flows and travel corridors, and provide evidence of new travel patterns caused by shifts in the location of employment or new housing developments.

Area TypeNumber of

Tracts

Number of Internal Tract

Trips

Percent of Trips

Originating in Area Type

Average Internal

Trips Per Tract

Maximum Internal

Trips

LED

Urban 1,087 38,192 2.7 35 714

Suburban 907 89,033 4.7 98 657

Non-Metro Urbanized 94 16,967 16.1 181 780

Rural 752 129,607 14.1 172 1,338

Total 2,840 273,799 6.4 96 n/a 

Census

Urban 1,104 118,075 6.8 107 2,115

Suburban 910 185,480 8.4 204 1,880

Non-Metro Urbanized 94 26,420 23.4 281 1,010

Rural 754 237,659 19.4 315 1,890

Total 2,862 567,634 10.7 198 n/a