demographic response to transportation innovation: the case of the interstate highway

Demographic Response to Transportation Innovation: The Case of the Interstate HighwayAuthor(s): Daniel T. Lichter and Glenn V. FuguittSource: Social Forces, Vol. 59, No. 2 (Dec., 1980), pp. 492-512Published by: Oxford University PressStable URL: http://www.jstor.org/stable/2578033 .

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Demographic Response to Transportation Innovation: The Case of the Interstate Highway

D A N I E L T. L I C H T E R, University of Wisconsin, Madison G L E N N V. F U G U I T T, University of Wisconsin, Madison

ABSTRACT The development of a system of interstate highways in the U.S. has re-

kindled an interest in demographic responses to transportation innovation. In this paper we examine the relationship between date of completion of an interstate highway and changes in nonmetropolitan county population and employment characteristics during three time periods, 1950-60, 1960-70, and 1970- 75. We also attempt to explicate the underlying process by which access to a modern highway affects growth or net migration. This is accomplished by decomposing the association between presence of a highway and net migration into direct and indirect (through industrial employment change) components. Results indicate that interstate counties consistently maintained an advantage over noninterstate counties in net migration, proportion experiencing net im- migration (or a turnaround in net migration), and employment growth. Data also suggest that the positive effect of highways on net migration was most in evidence in less remote areas and that it operated largely by promoting employment change in service employment-both nonlocal and tourist-related. There is little evidence that the demographic effect of highways proceeded through expanded manufacturing which, in turn, promoted inmigration.

Innovation in modes of transportation has long been recognized as an important stimulus of societal change, with consequences for the spatial

*This research has been supported by the Economic Development Division, Economics, Sta- tistics, and Cooperatives Service, U.S. Department of Agriculture, and by the College of Agricultural and Life Sciences, University of Wisconsin, Madison, through a cooperative agreement. Analysis was aided by a Center for Population Research Grant (HD05876), to the Center for Demography and Ecology, University of Wisconsin, Madison, from the Center for Population Research of the National Institute of Child Health and Human Development. We also express our appreciation to Tim B. Heaton for his assistance in assembling the data.

? 1980 The University of North Carolina Press. 0037-7732/80/020492-12$02.10

492



Highways & Demographic Changes / 493

organization of society and general patterns of population redistribution (Hawley; McKenzie; Weber; Yeates and Garner). During the nineteenth century, the penetration of railroads into America's heartland promoted the spread of population to previously remote areas. Equally significant is the nodal growth that was stimulated as settlements sprang up at intermit- tent points along railroad routes. The friction of space has been further reduced during this century with the advent of the motor vehicle and surfaced roads. As roadways have penetrated the landscape and heightened the accessibility of places through a restructuring of time-cost distance, the possibility of dispersal of population and economic activity has been increased considerably. Particularly significant, for example, has been the part played by motor transport in the territorial expansion of the range of daily activities within metropolitan areas (Hawley; Tobin). Also, the functional interdependence and exchange among places, and between market centers and their hinterlands has been greatly enlarged (Garrison; Hawley; McKenzie). This has undoubtedly contributed to the emergence of an inte- grated national economic system.

The authorization by Congress during the mid-1950s for the development of a system of limited access highways linking major U.S. cities has served to renew our interest in the effects of transportation innovation on patterns of economic activity and population growth. At least superficially, such a system of highways appears to have considerable potential for channeling growth in nonmetropolitan areas. Indeed, this possibility has spawned a number of empirical studies (e.g., Fuguitt and Beale; Hansen; Lee et al.; Malin; Ringenberg and Fuguitt) and policy-oriented discussions related to this topic (e.g., Ballangee; Deakin). To date, however, evidence of a growth-inducing effect is equivocal. As will become apparent in the following section, this stems in large measure from (1) the regional or case study design adopted in many studies; (2) the failure to adequately link date of completion of a highway with subsequent growth for more than one period of time; (3) the failure to adequately control other variables that impinge on the relationship between highways and growth; and (4) the failure to specify a system of relationships among economic growth, population change, and accessibility to a controlled access highway. The present paper is a comprehensive attempt to overcome these limitations. We examine the relationship between date of completion of an interstate highway and changes in nonmetropolitan county population and employment characteristics during three time periods, 1950-60, 1960-70, and 1970-75. In addition, we attempt to explicate the underlying mechanisms by which access to a major highway impacts growth or net migration. This is accomplished by decomposing the association between presence of a highway and net migration into direct and indirect (through industrial employment change) components. Such an analysis should enhance our understanding



494 / Social Forces Volume 59:2, December 1980

of the dynamics of the linkage between highway location and growth in nonmetropolitan areas.

Previous Research

Although highway transportation is not generally regarded as a sufficient condition for economic development, most view it as a necessary factor for contemporary economic growth (Dickinson; Gauthier; Grossman and Levin; Hansen; Shafran and Wegman). Highways serve to alter the time- space ratio and the per unit cost of transportation, thereby facilitating the decentralization of industry, the expansion of trade areas, and the growth of the physical flow of people through an area. Hence the effect of highways on population growth can be regarded as an outcome of changes in industrial employment and improved economic conditions resulting from location on an interstate highway. Recently, a number of studies (e.g., Frisbie and Poston; Shin; Sly) have adopted a human ecological framework that conceptualizes population growth or decline as a response to adaptive shifts in sustenance organization-shifts presumably necessitated by a changing technology and environment. In this tradition, the development of a major highway network can be viewed as giving impetus to fundamen- tal changes in the sustenance organization or economic activity in nonmetropolitan areas, resulting in a demographic response, namely inmigration.

Proximity to a modern highway improves access to markets, resource inputs, and labor-each an important consideration in industrial location decisions. Indeed, an efficient network of highways plays a "vital role" in linking industrial plants with the rest of the economy (Smith). Conse- quently, it is not surprising that good highway transportation is often regarded as a major factor in the decision of firms to locate in a given area (Connelly and Meiburg; Kiley; Smith; Wheat b, c). Not only are highways thought to be important in attracting new industry, but also in encouraging the expansion of existing firms (Grossman and Levin; Kuehn and West). Therefore, it is expected that employment growth in manufacturing is related to accessibility to an interstate highway. Previous empirical research has yielded rather inconclusive results. For example, Wheat (a) compared manufacturing growth rates in U.S. interstate and noninterstate cities and discovered that the effect of interstate highways existed only east of the Mississippi River and on the West Coast. Till found that the county rate of change in manufacturing employment in the South was unrelated to presence of an interstate highway for the period 1959-69, net of the effects of size of the largest place in the county and distance to nearest SMSA central city. In an analysis of the relationship between highways and economic development in the Ozarks, Kuehn and West report that highways were a




positive, but not a major factor, accounting for variations in manufacturing employment. They conclude that the presence of highways is not a sufficient condition for economic development in the Ozarks region. Thus, despite a solid theoretical foundation, we lack conclusive findings on the positive impact of highways on industrial development.

Because the accessibility of a place is improved as a result of being located on a major highway, the "range of a good" is also expected to increase. Improved highway transportation serves to reduce the functional distance between goods and services and the consumers of those goods and services. That is, better highways enable individuals to reduce the time and cost of travel between places. Or, as stated by Hawley, "(i)mprove- ments in transportation ... permit a wider scatter of an interrelated population without loss of contact" (237). Consequently, it is anticipated that the marketing area of places located on an interstate highway will be enlarged, with implications for growth in basic trade and service employment. This inference appears to have at least some empirical support from previous studies. For example, Frey et al. found that highways were related to growth in retail trade, but there was little growth in wholesale trade and transportation-related establishments. Highway mileage also has been discovered to be positively correlated with level of trade and service employment in the Ozarks region (Kuehn and West).

Finally, counties located on interstate highways are expected to experience employment growth in personal services needed to accommodate increased volumes of travellers or tourists through the area. An increased volume of expenditures for items as motel lodging and meals appears to be a direct consequence of the growth of traffic (Gamble et al.). Also, highways make remote recreational and amenity areas more accessible, perhaps inducing growth in tourist-related industry in such areas (Kuehn and West). An increase in service stations and lodging units has been noted in places located on a major highway (Frey et al.).

The preceding discussion suggests that interstate highways have a growth-inducing effect on employment in manufacturing, commercial activity, and tourist-related services. This argument implies that the effect of highways on net migration is mediated by employment change. Further- more, the direction of causality is assumed to be from highways to net migration, rather than the reverse. This argument has been forcefully made by Wheat (c). He argues that the manifest function of the interstate highway system is to link major U.S. cities (with routes as direct as possible) without regard to the rapidity of growth in intervening localities. Others are less certain about causal direction, suggesting that interstate highways may have followed growth in that they were probably constructed along existing transportation corridors (Fuguitt, b; Fuguitt and Beale). Neither explanation is necessarily inconsistent with the other. Certainly, it is plausible




that growth begets an interstate highway which, subsequently, contributes to additional growth.

Most empirical research has assumed, implicitly or explicitly, that highways ultimately contribute to population growth. Indeed, most bivari- ate examinations indicate that highways are positively related to population growth (e.g., Frey et al., Malin, 1971; Wheat, c). But the failure in each study to control for the effects of salient ecological factors, such as size or distance to metropolitan center, renders causal interpretation tenuous, at best. Studies employing more elaborate multivariate statistical techniques, in which relevant ecological variables are included in the analysis, have yielded mixed results. For example, some studies report a positive effect of highways on growth or net migration (Humphrey and Sell; Irwin), even after other variables such as size of largest place or initial population size are controlled. However, in a study of minor civil divisions in Pennsylvania, Humphrey et al. note an insignificant relationship between proximity to a limited access highway and migration turnaround, once other demographic and ecological factors were taken into account. Although Hansen found that counties that were fast gainers were the closest to highways, much of the difference could be accounted for by proximity to metropolitan areas. Nevertheless, within counties adjacent to an SMSA, a highway effect was still apparent, but was less in evidence among nonadjacent counties. The finding that the growth-inducing effects of highways are conditional on nearness to metropolitan areas and/or local urbanization has also been noted by Fuguitt and Beale, Humphrey and Sell, and Lamb. These studies underscore the importance of being sensitive to the ecological context when inferring the nature of causality between interstate highways and population growth.

In sum, the presence of good transportation appears to be a necessary part of any adequate explanation of nonmetropolitan population growth generated by inmigration. This effect is posited to operate through employment change in manufacturing, nonlocal trade and services, and tourist- related activity. This model is schematically presented in Figure 1. In recog- nition of the influence of ecological characteristics on net migration and employment change, proximity to metropolitan areas and initial size of largest place are also included as exogenous variables in the model. They denote access to markets and agglomeration economies-factors which are related to industrial location and trade activity (Nourse; Smith). Distance to nearest SMSA, for example, has been found to vary inversely with levels of manufacturing, services, and retail trade (Bogue; B. Duncan). Likewise, proximity to metropolitan areas and initial size of largest place are related to nonmetropolitan population change (e.g., Fuguitt, a; Fuguitt and Beale; Tarver and Beale).



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498/ Social Forces Volume 59:2, December 1980

Data and Procedures

The basic units of analysis in this study are all nonmetropolitan counties in the conterminous United States. Using this unit of observation has been regarded as problematic in that more marked population changes may occur for smaller geographic units as a result of the presence of a major thoroughfare (Humphrey and Sell). This argument has merit to the extent that highways promote only intracounty shifts in population. However, any inmigration from outside the county to places straddling an interstate highway will also be reflected in rates of net migration for the county as a whole. Also, there are various data for nonmetropolitan counties that are not available for smaller subunits. Counties also constitute a manageable number of cases for research at the national level. Most important, however, counties are unique among substate units in experiencing very few bound- ary changes since 1950. Consequently, the benefits from using counties appear to outweigh the disadvantages.

A constant 1963 nonmetropolitan definition of counties is used throughout this study with county equivalent areas specially designated for New England. Each county is coded as being on or off the U.S. interstate highway system as of 1960, 1970, or as projected in 1974. Highway data are obtained from Rand McNally Road Atlases for 1960 and 1970. In- formation about highway construction since 1970 came from a map of the completed interstate highway system as of June 30, 1974, available in a Department of Transportation newsletter (U.S. Department of Transporta- tion). The following criteria are applied in the determination of interstate highway status at each date:

1. Counties were coded as being on an interstate highway if more than 1/2

the route was completed within the county. 2. If the interstate highway was on a border between two counties, both counties were coded. 3. The interstate highway had to be complete to the central city of a SMSA or across more than one county to a place with more than 25,000 population. A few exceptions were made if the highway was complete to the city except for a few short parallel state highway sections. 4. Other limited access highways were included even if not part of the national interstate system. Examples include the Garden State Parkway in New Jersey and the Kentucky State Turnpike.

The volume of net migration between 1950-60 and 1960-70 is obtained from tabulations for each county published in two Current Population Reports (U.S. Bureau of the Census, a, b). Net migration for 1970-75 is available from an annual series of county estimates (U.S. Bureau of the Census, c, d). In the present analysis, average annual rates of county net




migration are computed for each time period to achieve comparability (since periods are of varying lengths).

The U.S. Census of Population provides data on population size, and employment in manufacturing, services, and tourist-related industry for 1950, 1960, and 1970. For the purposes of this paper, employment in "lnonlocal services" includes those working in wholesale trade, financial institutions, and trucking and warehousing. The industrial sectors included in this employment index are obtained from a listing of nonlocal services provided in Duncan et al. and denote services that largely serve nonlocal populations or functions. This selection of services seems particularly ap- propriate given our earlier argument that interstate highways extend the geographical limits of trade and service areas. Employment in retail trade is not included in this employment index, not only because of its primarily local function (Duncan et al.), but also because of the correspondingly strong reciprocal relationship with net migration. That is, population gains via inmigration promote growth in retail trade activities-growth needed to accommodate the presumably increasing local demand for everyday necessities. In turn, this stimulates additional population growth or reten- tion. The reciprocity of this relationship poses problems of model specifica- tion in our path analysis. By including retail trade, we would overestimate the degree to which the effect of interstate highways on net migration was transmitted indirectly through this industrial category.

Included in our measure of tourist-related industrial employment are those employed in eating and drinking establishments, hotels and other personal services (excluding private households), and entertainment and recreation services. Rates of employment change between 1950-60 and 1960-70 are computed by taking the difference between the number employed in a given industrial category at the beginning and end of the period, dividing the result by the total civilian labor force at the beginning of the period, and then multiplying by 100.

Since past research findings indicate the importance of taking into account other ecological factors when examining changes in industrial employment or net migration, the present investigation employs proximity to metropolitan areas and local urbanization in the county as control variables. Proximity to metropolitan areas is measured by linear distance in 50 mile bands from the center of the county to the center of the nearest SMSA central city. Local urbanization is measured by size of largest place in the county,

The subsequent discussion proceeds, first, by presenting tabular data on the relationship between the completion of an interstate highway and changes in various population, net migration, and employment measures. Second, the parameters of the model depicted by Figure 1 are estimated using path analytic procedures. A decomposition of direct and indirect effects of interstate highways is undertaken in an attempt to understand




the underlying processes by which highways promote or retard migration to nonmetropolitan areas. (Details of the procedure are provided in Alwin and Hauser; 0. Duncan). Finally, because the effect of highways has been found to be conditional on access to an urban environment (Humphrey and Sell) and since highways have been suggested as a component of development strategies in remote areas (see Ballengee), we examine separ- ately the effect of highways on net migration for four types of counties. Each differs by proximity to nearest SMSA central city and initial size of largest place in the county.

Results

INTERSTATE HIGHWAYS AND POPULATION CHANGE

In Table 1, counties are grouped by date of completion of an interstate highway, and several measures of change are computed for each time interval. The first panel indicates that counties currently on the interstate system had a larger average population at each date than those located off an interstate highway (compare last 2 columns). Moreover, county population size appears to be related to the date at which the interstate highway was completed. Counties located on interstate highways before 1960 had the largest average population at each of the four dates considered. And, among noninterstate counties and counties in which an interstate highway was completed after 1960, the largest mean population for 1960 was in those counties where the highway was constructed during the 1960s. The implication is that interstate highways are laid down in nonmetropolitan areas with above average population concentrations, suggesting that interstate highways may have been constructed along existing transportation corridors and may be near metropolitan centers.

There is also evidence that interstate highways are constructed in counties that, in prior periods, experienced higher than average rates of net migration. Since data were unavailable for the 1940-50 period, infer- ences about the effect of higher rates of net migration on whether an interstate highway was constructed between 1950-60 are not possible. However, data for the later periods are available. As data in panel 2 of Table 1 indicate, among noninterstate counties and counties on an interstate subsequent to 1960, the highest average annual rate of net migration for the 1950-60 period was in counties with interstate highways built just subsequently between 1960-70 (-.963). Similarly, among noninterstate counties and counties on an interstate only after 1970, the average annual rate for 1960- 70 was highest in counties in which the highway was built after 1970 (-.644). Thus, the evidence suggests that interstate highways were constructed in areas already experiencing above average rates of net migration.




Table 1. NONMETROPOLITAN COUNTY MIGRATION AND POPULATION CHARACTERISTICS BY DATE OF COMPLETION OF INTERSTATE HIGHWAY

Date Of Completion Total Off

Before 1960 1960-1970 After 1970 Interstate Interstate

Mean Population

1950 54,585 29,173 21,434 29,416 18,504 1960 64,428 32,171 23,250 32,731 18,882 1970 74,989 35,862 25,382 36,636 19,667 1975 80,037 39,349 27,818 39,977 20,935

Average Annual Rate of Net Migration

1950-60 -.342 -.963 -1.231 -.979 -1.747 1960-70 .054 -.279 - .644 -.349 - .879 1970-75 .583 1.007 .801 .910 .534

Adjusted* Annual Rate of Net Migration

1950-60 -.830 -1.140 -1.270 -1.147 -.670 1960-70 -.330 - .380 - .650 - .450 -.830 1970-75 .670 1.080 .830 .972 .510

Proportion Gaining by Net Migration

1950-60 40.7 4 22.3 15.9 22.2 10.0 1960-70 54.3 38.Q 28.5 36.9 21.2 1970-75 59.3 72.9 69.9 71.7 61.1

Proportion Experiencing Turnaround** After Net Outmigration

1960-70 37.5 35.8 22.9 27.7 17.8 1970-75 54.1 62.7 62.6 62.1 54.0

N 81 539 239 859 1804

*These are the adjusted means derived from a multiple classification analysis in which distance from SMSA in 1963 and size of largest place at the beginning of the decade are introduced as covariates.

**The number of outmigration counties in previous time period is used as the denominator in the computation.

Evidence that interstate highways are completed in areas charac- terized by high population concentrations and above average rates of net migration does not preclude the possibility that interstate highways may also have a subsequent effect on net migration. First, we find that interstate counties maintained a consistent advantage over noninterstate counties with respect to net migration rates for each period (panel 2, columns 4 and 5). Moreover, counties in which an interstate highway was constructed during the 1950s had a higher average annual rate of net migration for the period 1950-60 than other counties ( - .342). In addition, the effect appeared to carry over to the subsequent decade as well. Among counties not on an interstate highway until after 1960, the highest rate of net migration for 1960-70 is expected in counties in which an interstate was built between 1960 and 1970. This expectation is confirmed by our data. A similar pattern




of association between date of opening and net migration is also apparent for the 1970-75 period. Thus, the highest rate for a given time period is generally found for counties in which an interstate was built during that period.

Moreover, these findings are not simply a function of the association of interstate highways with size of largest place in the county or distance to nearest metropolitan central city. Even when the average rate of net migration is adjusted for these variables in a multiple classification analysis (panel 3, Table 1), a similar pattern of association between date of interstate construction and rates of net migration is evident, although the effects are reduced somewhat. Although interstate highways were constructed in counties already growing more rapidly than other counties, location on the system nevertheless appears to make a subsequent contribution to differ- ential county growth through migration.

The effect of interstate highways is further illustrated by the larger proportion of interstate counties that experienced net inmigration during each time interval than did counties located off an interstate highway (panel 4, Table 1). Also, date of highway completion is related to proportion gaining by net migration in a manner similar to that described for the preceding panels of Table 1 which show mean rates. For example, the proportion of counties experiencing net inmigration for the 1950-60 period was largest (40.7 percent) among those counties in which an interstate highway was constructed during the 1950s.

Finally, counties with an interstate highway were more likely to record a turnaround in net migration (i.e., experience net inmigration after showing net outmigration during the preceding time interval) than were counties located off the interstate system. For example, among those counties located off an interstate highway that experienced net outmigration between 1950-60, 17.8 percent registered net inmigration for the period 1960-70. This compares to 27.7 percent for counties located on an interstate, regardless of date of completion (compare column 4 and 5, panel 5). Counties on the interstate system before 1970 experienced the largest percentage turnaround during the 1960s (37.5 and 35.8) as expected. Also, for the period 1970-75, counties on an interstate highway constructed after 1960 were the most likely to experience a turnaround in net migration.

We have demonstrated that interstate highways were built in areas distinguished by above average population concentrations, indicating that their routes may have tended to follow existing transportation corridors. Indeed, 73 percent of all counties currently on an interstate highway had a "principal through highway" as designated by Rand-McNally in 1949, compared to only 39 percent for noninterstate counties (data not shown). Thus, any effect of interstate highways on net migration might be expected to be diminished if built along existing transportation corridors; and accen- tuated in areas previously off a major transportation corridor. In Table 2,




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counties are cross-classified by the interstate highway variable and whether the county is on a principal 1949 through highway. Interstate highways had a positive effect on average annual rate of net migration for all three time intervals, regardless of whether they were on a major throughfare in 1949. However, for the 1950-60 and 1960-70 periods, the difference between on and off interstate counties with respect to average annual rates of net migration was greatest for counties not located on a principal through highway in 1949. Thus, consistent with expectations, interstate highways appear to have their largest impact on net migration in areas previously located off major transportation corridors. Note also that counties located off both a major highway in 1949 and an interstate highway experienced the highest rate of net outmigration for the 1950-60 and 1960-70 periods. By 1970-75, this pattern no longer obtained. As a result, the difference between rates of 1970-75 net migration for interstate and noninterstate counties is smaller for counties off 1949 principal highways than for counties on such highways. This may simply reflect a deterioration of our 1949 highways measure in that new highways (other than interstate highways) have undoubtedly been constructed after 1949. Or, more substantively, the finding may well reflect the post-1970 patterns of increased population redistribution to remote nonmetropolitan counties. The important point, however, remains that presence of an interstate highway still makes a difference in county rates of net migration, regardless of location on existing transportation corridors prior to interstate highway construction.

EMPLOYMENT CHANGE, NET MIGRATION, AND INTERSTATE HIGHWAYS

The presence of an interstate highway is also expected to have implications for the location of jobs. In Table 3 are gross and adjusted rates of employment growth in manufacturing, nonlocal services, and tourist-related industry by interstate highway status in 1970. The adjusted rates are derived from a multiple classification analysis in which distance from nearest SMSA central city in 1963 and size of largest place at the beginning of the decade are included as covariates. Examination of the gross rates suggests that being located on an interstate highway is positively related to employment growth in each of the three industrial employment classifications for both 1950-60 and 1960-70. Moreover, the effect appears to be getting larger over time as the difference between being on an interstate highway, and being off, is larger for 1960-70 than 1950-60 for each industrial classification. Furthermore, this pattern of association is not significantly altered after adjusting for the effects of distance from metropolitan areas and local urbanization. Thus, the data serves to illustrate the effect of interstate highways on changes in employment patterns in nonmetropolitan areas.

A major objective of this paper is to explicate, using path analytic techniques, the process by which access to the interstate highway system




Table 3. GROSS AND ADJUSTED* NONMETROPOLITAN COUNTY MEAN RATES OF EMPLOYMENT GROWTH BY INTERSTATE STATUS IN 1970

Gross Adjusted Industrial

Classification On Off On Off

Manufacturing

1950-60 5.45 3.96 5.11 4.06 1960-70 7.58 5.79 7.21 5.91

Nonlocal Services

1950-60 1.50 1.02 1.42 1.04 1960-70 2.17 1.37 2.08 1.42

Tourist-Related

1950-60 .74 .58 .81 .55 1960-70 1.71 1.02 1.73 1.01

*Adjusted means derived from a multiple classification analysis in which distance from SMSA in 1963 and size of largest place at beginning of the decade are introduced as covariates.

promotes inmigration. In Figure 1, the effect on net migration is posited to be a result of changes in manufacturing, nonlocal trade and services, and tourist-related commercial activity. Because industrial employment data are unavailable for the post-1970 period, and too few interstate counties (81) are available for detailed analysis for the 1950-60 period, we restrict our analysis to the period 1960-70. Interstate highway is a binary variable with counties dichotomized on the basis of presence of an interstate highway before 1970. Rates of employment changes and net migration are for the period 1960-70. Distance to nearest metropolitan central city in 1963 and size of largest place in the county in 1960 are employed as exogenous control variables.

In Table 4 are the total associations (r's), and the direct and indirect effects of interstate highways on county annualized rates of net migration for 1960-70. As indicated by the product-moment correlation coefficients and the partial regression coefficients, presence of an interstate highway is positively associated with rates of net migration (RMIG) and employment change. These findings corroborate the earlier data reported in Tables 1 and 3.

Although the indirect effect coefficients in Table 4 are small, they nevertheless indicate that the effect of interstate highways on net migration is mediated largely through employment changes in nonlocal services (RSER), and tourist-related (RTOu) activity. In fact, approximately 36 percent




of the total effect of interstate highway on net migration (obtained from reduced form coefficients; see note in Table 4) is due to employment change in nonlocal services. Furthermore, and perhaps surprisingly, only a small percentage (6.7) of the total effect is accounted for by changes in manufacturing (RMAN) employment. Thus, there is little support for the popular notion that highway development fosters industrial development which, in turn, stimulates population growth.

In Table 5, we report the effects of interstate highways on net migration within varying ecological contexts. The direct and indirect effects are obtained for each of four types of nonmetropolitan counties: (1) remote- rural: counties 100 miles or more from the nearest SMSA central city, with the size of the largest place less than 2,500 in population; (2) remote-urban: counties 100 miles or more from nearest SMSA central city, with the size of the largest place 2,500 or greater; (3) near-rural: counties less than 100 miles from nearest SMSA central city, with the population size of largest place less than 2,500; and (4) near-urban: counties less than 100 miles from nearest SMSA central city, with the size of largest place 2,500 or greater.

Results indicate that the effects of interstate highways on county net migration and employment change do indeed vary by the location categories defined above. The major difference is between the remote-rural counties and the other three more urban-like sets of counties. The total, direct, and indirect effects are negative for the 354 remote-rural counties (Table 5, panel 1), whereas those for the other county groupings are positive, with quite similar total effects ranging between .14 and .18. The rela- tive importance of the direct effects, however, is greater for the two groups of counties near metropolitan centers, perhaps reflecting a net migration gain through expanded commuting-a gain not captured by the specific employment categories considered here. Nevertheless, from 50 to 80 percent of the total effect on net migration in each county group is due to the indirect effects through growth in service and tourist-related employment.

The sizes of these effects are small and, therefore, should be inter- preted with care. Still the results indicate counties traversed by an interstate highway experienced a centralization of jobs and residences provided they were accessible to metropolitan centers or included nonmetropolitan urban places within their boundaries. We should note that a possible factor contributing to the negative relationships among less urbanized remote counties is that many of them, particularly those having recreational ame- nities (e.g., in northern New England, northern Great Lakes or Ozarks regions), have recently experienced renewed population growth despite being located off the interstate system.




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Highways & Demographic Changes I 509

Discussion and Conclusions

Historically within the United States, each improvement in transportation has had significant ramifications for both the spatial organization of the landscape, and patterns of interaction among widely dispersed populations. In recent decades, with our ever-increasing reliance on motor transportation, the construction of a system of interstate highways has rekindled an interest in demographic responses to transportation innovation. Indeed, access to a modern highway system has frequently been assumed to promote economic and population growth. The chief aims of this study have been to investigate the extent and direction of the "interstate effect;" and to explicate how this process operates through changes in various industrial employment sectors. Previous investigations have concentrated almost solely on the former, often speculating about underlying processes but failing to provide empirical evidence. The present paper was an attempt to fill this void by examining net migration in nonmetropolitan counties in the United States.

The question now appropriately asked is: Does it make a difference if a county is on or off the interstate highway system? Based on the evidence presented, there seems to be little question that access to an interstate highway has a modest impact on both economic growth and net migration. Although there is evidence that highways were built in areas already experiencing a high level of economic development and growth, counties on the interstate system consistently maintained an advantage over noninterstate counties with respect to net migration, proportion experiencing net inmigration, a turnaround in net migration, and employment growth. Alone, these findings may not seem so convincing, especially since there is evidence that interstate highways were constructed in areas along existing transportation corridors. Yet the consistency of evidence provided on the relationship between date of completion of the interstate highway and various demographic measures was striking. Moreover, even after controlling for the effects of relevant ecological variables in a multivariate framework, the positive effect of interstate highways on net migration and employment change persisted.

Although highways contributed to growth in manufacturing employment the present investigation provides little evidence that the demographic effect of interstate highways on net migration proceeded through expanded manufacturing. For the U.S. as a whole, our data suggest instead that the effect of interstate highways operates largely by inducing employment change in service activity-both nonlocal and tourist-related. This suggests that interstate highways have the effect of redrawing trade or service areas, with implications for growth in employment in interstate counties. The effect on net migration operating through employment



510 Social Forces Volume 59:2, December 1980

growth in services seemingly reflects the competitive advantage counties on an interstate highway have in attracting consumers from outside the immediate area. The demand for such goods presumably increases the demand for labor which is translated into reduced outmigration or even inmigration.

As in previous studies (Hansen; Humphrey and Sell), the positive effect of an interstate highway was found to vary inversely with distance from a metropolitan area. It is noteworthy that the rate of net migration in remote rural nonmetropolitan counties was not affected positively by the presence or absence of an interstate highway. Thus, although remote areas are frequent targets of regional economic development efforts, the evidence presented here indicates a migration advantage of the interstate highway system only among counties with urban places. Although highways appear to play a modest part in economic and population growth in certain areas, whether the supposed benefits warrant a massive infusion of federal or state funds for highway development remains a matter of policy debate.

In sum, it appears that the interstate highway system, as with previous transportation innovations, has had the effect of altering the time-cost distance which underlies accessibility. The resulting diminution in the friction of space is seen as having a significant impact on population redistribution in nonmetropolitan areas, particularly in terms of channeling economic and population growth in less remote areas transversed by an interstate highway. Access to a modern highway network thus appears to shift the nature of sustenance activities in nonmetropolitan areas, which is reflected in the ability of areas to retain their population or attract new residents. Only time will fully reveal the impact of this demographic process on population redistribution in nonmetropolitan America.

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