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Chapter III

TRAFFIC PATTERNS, 1980

In order to make this 1980 forecast of maxi­mum effectiveness for route planning, it is essen­tial to know more than just the number of trips predicted and their terminal points. The direc­tion and length of the trips must be known. By forecasting the zone-to-zone · movements, the future trips can be described in terms of link­ages between origin and destination zones and the amount and type of future needs for serv­ice established. Both the present and the future traffic movements must be directed over pro­posed new systems in order to test the kind of traffic service these new roads will provide. Sub­sequent parts of this chapter will be concerned with the estimation of the 1980 zone-to-zone traffic movements and the illustration of the re-. suits of this forecast.

Procedure For Projecting Traffic Movements

It is obvious that planning based solely on current conditions is not sound procedure and for this reason traffic analysts have directed serious effort toward the problem of projecting travel movements.

FLAT RATE EXPANSION

Different methods for estimating future pat­terns (or for bringing older surveys up to date) have been developed. The first and simplest was to increase all inventoried movements by an average rate of traffic increase. Since this method expands all parts equally, there is no change in the relative magnitude of movements between pairs of zones and traffic growth rates at all zones will be identical. This method was soon discarded because it was inconsistent with the known tendency for parts of urban areas to grow at greatly varying rates. This would obviously produce differential growth rates in

28

zones. For example, the vacant lands in sub­urban areas which are newly developed will have far greater percentage increases in trip origins and destinations than the relatively stable, older areas which are already fully de­veloped.

AVERAGING ZONAL GROWTH FACTORS

To overcome this obvious difficulty, traffic analysts divided study areas into a series of zones and computed separate growth factors for each zone. With this improvement, it be­came clear that traffic movements between zones of high growth would increase more rapidly than movements between zones having little or no growth. However, the exact amount of increase in zonal interchange was not known because two zones were involved in each traffic movement. Therefore, the average of the two zones' growth factors was used to change the known trip volumes. Fratar1 makes a valid criticism of this method. He points out that it is inconsistent with the premise that the traffic growth factor of a zone can be estimated. This inconsistency results from the fact that the trips terminating at each zone after the procedure has been applied, will not equal the totals which were predicted for that zone. The zone of high­est growth will not have as many trips as pre­dicted and zones of lowest growth will have more than the predicted number.

COMPARATIVE GROWTH FACTORS AND ITERATIONS

If the method of forecasting trip terminal volumes at zones as described in Chapter II above is sound, then Mr. Fratar's criticism of the prior methods of trip forecasting is valid. Many of his suggestions were used in devising

lFratar, Thomas J., "Forecasting the Distribution of Inter­zonal Vehicular Trips by Successive Approximations", paper delivered at the Highway Research Board, January, 1954.

the method for forecasting zone-to-zone move­ments in the Detroit Area. This procedure in­volves continual readjustment of traffic inter­change volumes until the trips entering and leav­ing each zone are in balance with the predicted trip totals for each zone.

Much of the logic as to how trip forecasting should be done was developed in Chapter V of Part I. It was demonstrated in this part of the Report that travel volumes between any pair of zones would be proportional to the prod­uct of the number of trips terminating at each zone and inversely proportional to the "friction of travel" between those zones. It was further shown that the friction of travel as it was de­fined and measured could be approximated by measures of travel distance. Since the Study Area is uniformly fiat and quite evenly supplied with roads, distance and time of vehicular travel are presently highly correlated. It is reasonable that improvements in travel time between zones (for example the construction of expressways) wili not be so selective as to highly favor travel between selected pairs of zones at the expense of others. In brief, it is reasoned that improve­ments in traffic facilities will be evenly dis­tributed in any future plan so that the friction between zones will remain relatively the same in 1980 as in 1953, i.e., roughly proportional to the travel distance. Thus, the logic for trip forecasting concerns the future number of trips into and out of any zone and the friction be­tween zones as defined by the 1953 survey. These two factors provide the basis for the predictive formula used.2 This formula is differ­ent from that suggested by Fratar but his

2The formula used was Y;i'=Yii (GF; ~ GF,) Where Y;j' represents the number of trips between any

zone I and any other ~one J; YIJ is the old number of trips between I and J; GF, IS the growth factor of zone I that is ~he t~tal t~p. ends_ at zone I at the beginning of any ~omplet~ 1terat10n dJVJ~ed mto the predicted 1980 total trip ends at zone I; GFJ IS the comparable factor at zone J; and K is the same kind_ of factor for all trips in the system. . A detailed procedural write-up and the machine calculat­mg procedures are contained in a separate technical paper :'The Method of Forecasting Zonal Interchange Volumes Used m the Detroit Traffic Study," 30 pp. mimeo., October, 1955.

iterative or smoothing procedure was the neces­sary clue needed to bring the system into a bal­ance consistent with the predicted trip totals at zones.

SPECIAL PROBLEMS TO BE SOLVED BEFORE MAKING THE ACTUAL FORECAST

Several technical problems had to be solved before this formula could be applied. These preliminary problems are described in the fol­lowing sections for the more technical reader.

Zones of Unusually Great Growth

Some suburban zones which were largely va­cant in 1953 had very high growth factors . Six of these were expected to grow by more than ten times in trip production. This presented a major problem because the small number of trips inventoried in 1953 (as few as 3,000 terminating at one zone) would not adequately represent all possible zonal interchanges. Their travel interchange with many other zones was zero in 1953 and would continue to be zero ac­cording to the formula. Actually, no pair of zones can logically have zero trips on an aver­age weekday because it is practically certain that on at least one day in a year a trip will be made between each possible pair of zones in the Area. If this occurred, (i.e., one trip was made in a year) then there would be about 1/ 250th of a trip on an average weekday. Since the smallest sample size for internal trips was one in ten, the expansion of the sample allowed the smallest positive count of ten trips between internal zones. Therefore, there were many pairs of zones with no trips recorded between them. To reduce the number of zero inter­changes for the six zones with the highest growth factors, increased trip interchange of a normal character was built up between each of these zones and all others.

This addition of trips was accomplished by first making a listing of all of the 264 possible travel interchanges for each of the six problem zones. For each problem zone, the 264 pos­sible interchanges were arranged according to

29

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travel distance. Using the developed curves which were illustrated in Chapter V of Part I/ enough trips were added to bring positive values of vehicular trips at all possible inter­change combinations. To do this, it was neces­sary to add quite a few trips at each of these six zones and therefore to the system of trips.

After these trips had been added to the total inventoried trips, new growth factors were com­puted and no zone had a growth factor above 10. There were, of course, many additional pairs of zones with no interchange represented. While a good case could be made for making similar corrections in these instances, there were a sufficiently large number of trips at both terminal zones so that such a procedure would not seriously have changed the final results and would have been extremely time consuming.

Prediction of 1980 Volumes of External Local Trips

Since there were no land data for the outside terminal points of external trips, growth fac­tors for external trips had to be computed in a different way. First, the Cordon Line was divided into ten sections and all stations in each section were grouped.4 Growth factors were then computed for each of the ten groups of ex­ternal stations. To compute these growth factors, it was reasoned that the external trips are sub­ject to the same predictive laws as internal zone­to-zone trips . To predict these in the same way, however, would involve creating growth fac­tors for the entire United States! This was clear­ly impossible. The number of export-import trips made is obviously a function of the traffic generating power of the internal area. That is, the larger and more important the Area en­closed by the Cordon Line, the more trips will be made to it and, by the same token, the more trips it can export. Therefore, growth in the

3These are further detailed in the Technical Report, "An Analysis of the Factors Affecting Zonal Interchange Volumes," 17 pp. mimeo., Detroit, August, 1955, by Howard W. Bevis.

4Stations were grouped according to sectors of the Cordon Line. See key Map 29 in the Appendix of Part I.

30

number of external trips would be most re­liably estimated by using the growth of the in­ternal area. More outside residents, for ex­ample, would be expected to come into the De­troit area as there were more people and ac­tivities in the Detroit area to attract them. Cor­respondingly, residential growth inside the Area would increase the number of potential trip makers who could leave the Area.

Research has shown that the number of resi­dents crossing the Cordon Line (if this line is located far enough out to contain the bulk of local movements) is always nearly the exact complement of the number of non-residents crossing. In brief, the analysis of external traffic showed that not only did the inbound non-resi­dents equal the outbound, the residents out­bound and inbound tended to equal the direc­tional non-resident volumes. This held for trucks, passenger cars and people.

The bulk of the trips interruped at external stations had an outside origin or destination close to the Cordon Line. Only 15% of the external trips had one end of the trip outside of the surrounding seven counties. Also, many of the trips interrupted were short trips termi­nating just inside the Cordon Line. Therefore it was reasoned that the zones near the Cordon Line would have a greater influence on these trips than the inner traffic zones. In order to properly weight these differences, each station group's trips to each internal zone were multi­plied by the growth factor at that zone and the resultant totals were summed for each station group.5 The totals derived constituted the es­timated 1980 volumes for that station group and were the basis for determining the growth factors.

Through Trips

Through trips were factored by a general traffic growth figure for the State of Michigan ( 1.7) and played no further part in the de-

5This logic is developed and proved in the separate technical paper, "The Method of Forecasting Zonal Interchange Volumes Used in the Detroit Traffic Study," op. cit .

• I

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velopment of zone-to-zone or zone-to-station forecasts. While this was a somewhat arbitrary procedure, the number of through trips was so small ( .2% of the total vehicular trips) that additional, refined estimates were not war­ranted. Also, it is quite reasonable to expect that if all traffic in Michigan grew by 70% in 25 years, trips through the Study Area would increase by about the same amount.

Correction for Screen Line Check Factor

. It will be recalled that all comparisons of the 1953 survey data with actual ground counts showed that twenty-four hour traffic volumes by expanded survey data accounted for slightly more than 80% of actual ground counts. At this point it was essential that the 1953 zone-to­zone volumes be increased to equal the ground counts. Otherwise the amount of traffic demand would be substantially understated. All internal trips were expanded by a factor of 1.2. This would give a 100% representation if 83.3% of the internal trips were represented by the orig­inal expansion of the home interviews. In the light of all evidence, this was considered as a very reasonable procedure. This fiat increase of all trips over-emphasizes travel to work places and under-emphasizes off-peak travel and trips of a social-recreational character. All available evidence shows the former were most reliably reported at homes and the latter most frequently under-reported. Even if work trips are over­emphasized in the total twenty-four hour pat­tern as a result of using a constant factor, no great harm is done because these are the most critical trips for which a system should be planned. Since work trips are not concen­trated in any single zone in the Detroit Area, possible travel distortion logically would be very small.

The 1980 Forecast Of Traffic Movements

After all of the preceding work had been done, there were two known factors from which

the future travel pattern could be derived. First, there was a complete and known pattern of inventoried 1953 traffic movements Second­ly, future expected trip totals had been pre­dicted for every terminal zone or station group.

To develop the desired new pattern, the vol­ume of traffic movement between each pair of terminal points was adjusted in accordance with the amount of change predicted at each terminal point. This was accomplished for each of the 34,574 possible movements by use of the previously described predictive formula. The process adjusted all travel volumes from the in­ventoried system and thereby created a new sys­tem of movements more consistent with the pre­dicted trip totals at each terminal zone or station group.

The new traffic pattern thus obtained did not, however, meet the condition of having the pre­dicted number of trips terminating at each zone or station group. This occurred because the zonal totals could only be determined after each interchange had been independently revised and these new interchange estimates re-sum­marized at terminal zones.

Therefore, the process was repeated using the newly predicted traffic pattern as the new point of departure. This complete repetition of calculation is known as an "iteration". In ac­tual practice, five complete calculations or itera­tions were made before the trip totals at each terminal point which were derived from the pre­dicted system of traffic interchange closely ap­proximated the totals predicted at each of these zones and station groups.

After the last iteration, all zonal trip totals were within plus or minus 3% of the predicted 1980 totals and only 18 of the 264 zones and station groups showed a difference in excess of 1%.6 Further iterations would obviously not change the results appreciably and therefore the distribution of trips after the fifth iteration was considered as a reasonable approximation of the

6Zonal volumes after each iteration are listed in the Appen­dix, Tables 16 and 17, to demonstrate the actual results of the briefly described process.

31

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traffic movements of which would occur when the total traffic generated had reached the pre­dicted volumes at each terminal point.

The results of this expansion and forecast are of substantial importance in determining the adequacy of any plan since the travel so de­veloped is the principle measure of need for roads. The critical reader will ask, "How do these future trips compare with the present pat­tern and how reasonable are the results ob­tained by these seemingly rigid mathematical propositions?"

In the preceding Chapter the total trips of the future were determined. Therefore, one must look to the characteristics of the forecast trips in order to determine whether the results of this iterative procedure are consistent with knowl­edge of traffic behavior.

COMPARISON OF 1953 AND 1980 TRIP CHARACTERISTICS

There are many possible ways to classify trips according to their characteristics. Of predomi­nant concern to the effectiveness of this Study is the projection of vehicular trips.7

Two characteristics of vehicle trips are of par­ticular concern in determining route needs. The first is the length of future trips and therefore the miles of travel. The second is the pattern of vehicle trip desire, which indicates the loca­tion of needs.

7Detailed district-to-district tables showing both 1953 and 1980 projected volumes are shown in the appendix, Table 18.

Trip Length 1953 and 1980

To compare the future forecast with the 1953 inventory, all possible traffic movements using zones and station groups were classified accord­ing to the distance via the shortest surface ar­terial street from the mid point of one zone or station group to another. Table 4 was then made up comparing trip volumes and vehicle mileage inventoried in 1953 to the same meas­ures predicted in 1980.

This table shows quite reasonable growth in trips of all lengths. The percentage distribution of trips and of mileage by travel distance is not severely altered in 1980. However, there are changes. There is a greater proportion of long trips in 1980 and an increase in the average trip mileage. This is to be expected. The great in­crease in suburban growth necessarily increased vehicle trips where journeys were longer than average. The suburban zones, having lower than average density of development, simply have greater average distances between possible traffic origins and destinations. Also, the in­creased exchange of trips between outer sub­urban zones serves to increase the number of longest possible trips which travel from one side of the Study Area to the other. The external trips which increased by a greater than average amount, are heavily weighted with longer trips and this also helps to produce the results shown in Table 4.

TABLE 4 COMPARISON OF VEHICULAR TRIPS AND VEHICULAR MILES ACCORDING TO TRIP LENGTH 1953 AND 1980

Arterial Street Distance Total Vehicle Trips Total Vehicle Miles

1953 Percent 1980 Percent 1953 Percent 1980 Percent

o- 2.9 miles ............. 2,170,881 47 .3 3,460,131 45.2 3,395,573 14. 7 6,007,498 14.8 3- 5.9 miles .... ........ . 1 ,080,766 23.6 1,932,461 25.1 4,664,989 20.2 8,270,704 20.4 6- 8.9 miles ..•......... . 573,798 1.2.5 952,169 12.4 4,233,076 18.4 7,001,333 17 .4 9-11.9 miles ....... •.•• .. 336,651 7.4 561 ,181 7.4 3,487,114 15.1 5,819,806 14.4

12-14.9 miles .......... ... 172 ,710 3 .8 302,992 4.0 2,307,286 10.0 4,057,440 10.0 15-17.9 miles ...... .. ..... 103,971 2 .3 194,781 2.5 1,704,306 7.4 3,196,789 7.9 18-20.9miles ...••..... ... 56 ,432 1.2 106,354 1.4 1,095,206 4.7 2 ,063,696 5.1 21+ • ••......••........... 84,867 1.9 156,914 2. 1 2,163,037 9 .4 4,041,159 10.0

Grand Total. •.. . .. .. 4,580,056 100.0 7,666,983 100.0 23,050,587 100.0 40,458,425 100.0

32

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It is quite reasonable that there should be some increase in average travel distance. The principal point of this table, however, is that trips in the 1980 forecast differ relatively little in their distribution by distance from those of the 1953 inventory. This fact supports the rea­sonableness of the forecast because no radical rearrangement of the frequency distribution of trips by travel distance should be expected.

Comparison of 1953 and 1980 Vehicle Trip Desire Charts

While the reasonableness of travel distances was shown, the major test of the 1980 trip fore­cast is in the nature of the resultant travel pat­terns. To compare travel desire patterns of the vehicle trips for both points in time, two maps were prepared in identical fashion using iden­tical scales.

These maps were prepared by tracing all ve­hicular movements between all possible combi­nations of terminal zones and station groups across a one mile grid system.

Desire line movements of vehicles were traced from origin to destination and the number of desire lines passing through each mile square of the grid summed. Square mile areas of like de­sire line density were then grouped by the method of isolines previously described. These two maps show the density of vehicular travel desire per square mile at all points in the Study Area. Map 3 and Map 4 are placed on oppo­site pages for ease of comparison. Both maps have been greatly simplified to show only the major patterns.

The first impression from viewing these two maps is that the 1980 pattern is quite like that of 1953 only spread out over more territory. This is exactly the case. The 1980 patterns on Map 4 retain the same structural outlines as those shown on Map 3. Both maps are domi­nated by the core of highest traffic densities in the central part of Detroit just north of the Cen­tral Business District. The essential difference in 1980 is that there is more traffic everywhere.

This is reasonable from the patterns of growth illustrated in the preceding chapter. The large growth in the outer suburbs has worked to in­crease traffic densities in those areas by the greatest percentage. But the increased growth of population and activities has, of course, in­creased traffic densities to some degree every­where. Very simply, more people and more cars mean more traffic.

The patterns of travel in 1980 are quite meaningful and can be explained. The high density of vehicular desire near the center of Detroit comes about because the highest trip generating densities are expected to continue to be in and near the heart of Detroit. These central areas of intense commercial develop­ment have tremendous power to concentrate traffic. This is the primary cause of the high­est desire line densities. The desire lines of trips going to central areas are, of course, supple­mented by those of trips with origin and desti­nation on opposite sides of the dominant pat­terns since these "through lines" also contribute to the final densities shown.

In 1980 greater segments of the area are ex­pected to be subjected to high vehicular vol­umes. The red colors cover much more of the city in 1980. The blue areas, identifying desire densities from 60,000 to 100,000 vehicle trips per day per square mile, extend well outside of Detroit into the adjacent built-up area. The forecast results show that the traffic desire densi­ties of 60-79,000 vehicle trips per square mile which characterize areas in the City of Detroit like those near Six Mile Road and James Couzens Highway or at Harper and Connor will, by 1980, be found in the suburban areas of Inkster, Berkley or East Detroit. Other similar areal comparisons can be made to help com­prehend the future volumes resulting from the expected great increase in vehicular travel.

To further detail expected changes by 1980, two maps have been prepared. Map 5 shows the percentage increases in traffic desire densi­ties per square mile. This map is prepared by

33

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622 623

633

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672 673

671

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694 6 93

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LEGEND

This chart shows the pat­

tern of travel desire of

3,694,828 interzonal or

station group vehicle trips

in 1953. A total of 18,509,-

364 airline miles of vehic­

ular travel are traced on

this chart. All desire lines

are traced through a one

mile grid system.

Desire line traces per square mile

- 140,000 to 148,000

- 120,000 to 139,999

D 100,000 to 119,999

80,000 to 99,999

D D D

60,000 to 79,999

40,000 to 59,999

20,000 to 39,999

OHROIT MHROPOliTAN ARfA TRAR=IC STUDY 1955

DETROIT TRAFFIC STUDY AREA

TRIP DESIRE CHART

ALL VEHICULAR TRAVEL BETWEEN ZONES OR STATION GROUPS - 1953

34 Mop 3 ..

•

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LEGEND

This chart shows the pattern of

travel desire of 5,839,405

interzonal or station group

vehicle trips in 1980. A total

of 31,656,901 airline miles of

vehicular travel are traced

on this chart. All desire lines

are traced through a one m ile

grid syste!ll.

Desire line traces per square mile

• 160,000 to 162,000

• 140,000 to 159,999

120,000 to 139,999

0 100,000 to 119,999

- 80,000 to 99,999

CJ 60,000 to 79,999

0 40,000 to 59,999

CJ 20,000 to 39,999

OfTROIT MfTROPOLITAN ARfA TRAffiC STUDY 1955

DETROIT TRAFFIC STUDY AREA

TRIP DESIRE CHART

ALL VEHICULAR TRAVEL BETWEEN ZONES OR STATION GROUPS- 1980

35 Map 4.

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dividing the net increase in volumes by the 1953 volumes at each grid square to compute the percentage increase in trip traces by 1980. !so­lines are then used to group areas of like per­centage change.

Map 6 shows the actual amount of traffic de­sire gain at each square mile on the grid system. This map is made by simply subtracting all of the values on Map 3 from those on Map 4 and plotting the remaining values. Again isolines are used to group the areas of similar change in amount of traffic gain.

Together these two maps detail the location and degree of change from 1953 to 1980. Map 5 reflects the previously described growth in the suburban areas. The farther from the center of town, generally the greater the percentage in­crease in desire densities. The central, built-up areas show relatively minor percentage changes. This is expected because there is little room for any substantial increase where the land is completely built up.

When absolute amounts of change in traffic are shown on Map 6, the picture is somewhat

Per cent increase

s 500 and above D 250 to 499 D 100 to 249 D 50 to 99 D 10 to 49

PERCENTAGE INCREASE IN DENSITY OF VEHICU­LAR TRIP DESIRE LINES PER SQUARE MILE-1953 to 1980

Mop 5.

36

different. Here, quite obviously traffic has sim­ply increased everywhere. Increases are quite uniformly spread considering the great range in densities of Maps 3 and 4. Some reflection will show that this is quite reasonable. The very minor percentage increase in the densely built­up central areas still means many more trips be­cause of the already · high volumes existing.

The most densely built-up area near the cen­ter of Detroit did not show as great an absolute amount of traffic increase as the closer suburban areas. This, too, is expected because these inner areas are so densely developed that very little increase in traffic generation can be expected. However, as the nearby suburban areas become more densely developed, they have increased volumes both from the traffic they generate and from the traffic crossing their area between the central city and the fast growing outer suburbs. Therefore, these are the locations where abso­lute amounts of traffic desire should show the greatest increase.

This somewhat detailed illustration of the 1 980 forecast is made to demonstrate the traffic

Net increase

0 30,000 and above D 20,000 to 29,999 D 10,000 to 19,999

AMOUNT OF INCREASE IN DENSITY OF VEHICULAR TRIP DESIRE LINES PER SQUARE MILE-1953 to 1980

Map 6.

effects of such growth and to test the reason­ableness of the forecast. In all cases the results are quite reasonable and in accord with expecta­tions from available knowledge of traffic be­havior.

Summary Measures of the 1980 Forecast

Table 5 below brings together the summary measures of the 1980 forecast. It shows that auto ownership increases faster than population, vehicle trips faster than auto ownership, and vehicle mileage faster than vehicle trips. Vehicle mileage, the principal measure of demand for facilities, is shown to grow more than half again as fast as population.

TABLE 5 COMPARATIVE TOTALS-1953 INVENTORY WITH 1980

FORECAST

1953 1980 Percent Change

1. Population .•. •• .•.•.• 2 ,970,000 4,400,000 48% 2. Auto ownership • •• ••.• 854,000 1,370,000 61 "/o 3. Person trips • .• •••.•. • 6,895,829 11,401,456 66% 4. Vehicle trips •.• • •.. .• 4,580,056 7,666,983 67% 5. Vehicle miles via

shortest arterial street route ... . ..••.. 23,050,587 40,458,425 76Cfo

This greatly increased amount of vehicle mileage is principally due to the fact that growth in the Area means growth in the suburbs and these are the sources of high car ownership and greatest typical vehicular mileage driven. The greater-than-average number of miles per trip by residents of these areas is caused by the fact that the suburbs are farthest from the spe­cialized centers. Also this growth increases the long trips between suburbs which are on oppo­site sides of the city.

It is, of course, reasonable to speculate whether twenty-five years from now the resi­dents will still be using private autos in much the same way as they are today. What about individual air travel and what about other im­pending technological improvements in trans­port? What about car design and perform­ance? No one can tell for sure. But looking back 25 years to 1930, it is not unwarranted

to devise this kind of travel forecast. It is still extremely likely that people will have to move about to an extent comparable to that pre­dicted. And, finally, the mere measure of the magnitude of the present trend is essential in order to give some scale to the needs of the future and the appropriate kinds of plans for this anticipated future. Clearly this forecast must be subjected to continual review and re­appraisal. The methods for this are simple. They are based on the known sources of trip generation and upon orderly laws which de­scribe traffic interchange. The precise pattern will be different in 1980 but the degree of differ­ence is under control by two facts: first, more than two thirds of the land development for 1980 is already in place and the available land for expanded development is known; and sec­ondly, the increase of 1980 traffic is added to a carefully inventoried 1953 base.

DIRECTIONAL VOLUMES, 1980

When all interzonal vehicular movements have been traced out as they were on Maps 3 and 4, it is possible to read much about the density of travel desire. If one knows the area and its traffic pattern, one can also read some­thing of the direction and intensity of this travel desire. It is possible to estimate the vari­ous streams of traffic desire which make up the composite picture. However, in attempting to discern the future needs of the Area it is im­portant to know much more about the the direc­tion and density of traffic desire than can be estimated from the composite map. To supply this information all travel movements of 1980 were classified into one of four possible direc­tions of desire line movement. Separate maps were constructed for each direction of travel so that it would be easier to relate these traffic movements to possible future routes . Also the separate directional travel makes it easier to relate this desire to the capacity measures of the existing arterial street system. This is done in Chapter IV.

37

-...

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.,

These four, separate directional maps of the predicted 1980 travel are shown on the follow­ing pages. Each movement between zones or station groups in that direction is traced across a mile grid system. Trips are assumed to terminate at the mile square in which each zone center or station group falls. Each movement is classified into one of four possible travel di­rections. The trace values registered on the map from any single zone-to-zone movement are proportional to the number of trips making that transfer and the number of squares through which that movement is traced from origin to destination, i.e., the trip length. The maps sug­gest the various vehicle flows which would re­sult from movement over a straight line course from origin to destination.

Direction A-1980

Travel in Direction A represents all vehicular trips whose desire lines angle between straight north-south and northeast-southwest (one quar­ter of all possible trip angles). The heaviest volumes are seen to focus on the approach to the central business district from the northeast. The use of zone centers tends to create two "fingers" of blue in the northeast section of the Area as local movements between adjacent cen­ters are reinforced by the longer trips towards the heart of Detroit. The principal alignment of desire in this direction is from the lower south portion of the Study Area northeast through the central business district and up to­ward Mt. Clemens at the northeast corner. This alignment is dominant because of the focusing power of the dense commercial core of Detroit, but it is reinforced by the concentrated external traffic at South Telegraph Road and the heavy local traffic at Wyandotte (the small, blue "island" of high desire") .

There are other well defined movements. The dark yellow isolines of high density on the west side of the city. result from the concentrat­ing power of the Ford Rouge plant which draws travellers from northwest Detroit and west

38

Dearborn. A similar dark orange strand of travel desire can be seen extending northward from the city along the Mound Road axis. This is due to the intensive expected industrial development northward from Detroit along the New York Central Railroad right of way which runs between Mound and Van Dyke Roads.

The independent pattern at the western edge of the Study Area derives from the forecast and is based upon a very great increase in volumes which reflect the present behavior of travellers. The residents of these outer areas now travel to Farmington (Grand River and Nine Mile Roads) for shopping and other services. The great residential growth expected in this area by 1980 is predicted to reinforce this existing habit in such volume that a separate, north­south pattern of desire is shown there. The extensive growth and development of the land in and around Farmington appears to be near certainty by 1980. If this does occur, these kinds of travel patterns are not unlikely.

Direction B- 1980

Direction B covers all trips whose direction of travel is between a north-south and north­west-southeast direction. As was the case in 1953, the Woodward Avenue axis is the area of high density travel in this direction. This is inescapable because of the already great development northwest from the city center along the Woodward axis. This intense develop­ment generates such high traffic volumes that the maximum desire density in this direction must closely parallel Woodward A venue.

The greatest densities of any of the four direc­tions occur on this map. It has the most area in red (at desire line densities in excess of 50,-000 trips traces per square mile). The red area approaches the Central Business District but does not reach it. This occurs because of the tracing method used. The trace values at terminal points are halved in order to properly weight length of desire lines. This red area is then caused by the trips terminating at the

l

Central Business District and correctly places the maximum traffic weight just north of the CBD.

The next heaviest desire line densities (the area colored blue) lie principally to the west of Woodward Avenu~. A very marked exten­sion of the blue area is found north of the city limits in Oakland County. This results from the local traffic generation of north Wood­ward communities reinforcing the concentrated external-local movements through station group 74 which has the greatest volumes of any external group. There is a marked extension fingering down to the River Rouge industrial area. There is also a small concentration result­ing from the attraction power of industrial plants near Conner and Mack. The principal area covered at this density results from the great attracting power of intensively developed land all long Woodward Avenue. This develop­ment serves to combine and concentrate travel in this direction.

Finally, it is possible to see new strands of trip desire in light yellow in suburban areas which would not have been visible in 1953. These show desire line densities in excess of 10,000 vehicle trips but less than 20,000. To the west, it appears that good north-south roads will be needed to meet this growing demand. The same comment will apply in the north and north­east suburbs where these desire movements re­inforce those shown in Direction A. This con­dition is noticeable along the Mound Road axis (south from station group 72).

Since a north-south or east-west road must serve travel in two directions, a density of travel desire of 10,000 vehicles per square mile (the yellow color on any of these maps) suggests that divided four lane highways spaced one mile apart will be required to furnish adequate capacity. This is one way to evaluate the significance of the areas colored yellow. The reasoning further suggests that territory con­taining desire densities above 20,000 represents possible location points for expressways.

Direction C- 1980

Desire lines varying from northwest-south­east to due east-west are shown on this map. As in prior maps, the Central Business District tends to focus the desire lines. In this direction the high density desire is concentrated north of Grand River Avenue. This is in large meas­ure due to the expected intensive residential development in the northwest section of Detroit and on out into Oakland County. Also covered by the blue area is the travel desire which is . directed to and from the new Center Area at Woodward and Grand Boulevard. Surprisingly there is no great extension of desire out towards Farmington and Station Group 75. This is because greater growth is expected west of the Cordon Line near Plymouth than northwest out beyond Farmington. The external trips from the western perimeter of the Study Area ( Sta­tion groups 76-78) are expected to increase to a greater extent than travel on the routes grouped at location 7 5.

There is a noticeable, long extension of the 20-30,000 volume range across the east side of Detroit terminating at the heavily industrial­ized Conner-Mack area. This pattern illustrates one of the advantages of these directional maps. Such a movement could not be guessed from a look at the nondirectional map which sum­marized all vehicular desire in all directions. When directions are separated this crosstown movement is isolated and it suggests a high demand for service in this area in that direc­tion.

Direction D- 1980

The fourth directional isoline chart shows all movements oriented from due east-west to southwest-northeast. This chart should be read with the one for Direction C to evaluate due east-west needs and with Direction A to better understand desire in a southwest-north­east direction. For example, there is a very great east-west desire volume extending into the area from Station Group 76.

39

!i-! • . .!.' ' ....:.~.:

661

672

• !

' I

40

660

674

.... :

l

LEGEND

This chart shows the pattern of travel desire of 1,531,813 inter­zonal or station group vehicle trips in 1980 traveling in direc­tion "A"." A total of 8,748,172 airline miles of vehicular travel are traced on this chart. All d esire lines are t raced through a one mile grid system.

Desire line traces per

16y a ssumin9 that all trips begin at the southernmost end of the trip, trip desi re lines. ore classified into one of four poss1ble d~rec t1ons as shown below. Direct ion 11A 11 in­cludes due northeast a nd uo to but not includino due north directions.

N

"\JL'

D D D D

square m ile

40,000 to 43,000

30,000 to 39,999

20,000 to 29,999

10,000 to 19,999

OfTROIT MHROPOLITAN ARfA TRAFFIC STUDY 1955

DETROIT T RAFFIC STUDY AREA

TRIP DESIRE CHART

1980 VEHI<;:ULAR DESIRE LINE DENSITIES- DIRECTION A

Map 7.

•

681

••

680

••• •••

•••

DETROIT TRAFFIC STUDY AREA

TRIP DESIRE CHART

! I

LEGEND

This chart shows the pattern of travel desire of 1,450, 183 inter­zonal or station group vehicie trips in 1980 traveling in direc­tion " 8".1 A total of 7 ,394,257 airline miles of vehicu lar trave l are tra ced on this chart. All desire lines are traced through a one mile grid system.

Desire line traces per

18y assuming thot oil trios beQin ot the southernmost end of the tr ip, t rip desire lines ore classi f ied in to one of four possible di rections os shown below. Direction "8" in­cludes due north ond uo to but not including due northwest di rection.

-D D 0 D D

square m ile

60,000 to 63,000

50,000 to 59,999

40,000 to 49,999

30,000 to 39,999

20,000 to 29,999

10,000 to 19,999

DfTROIT MfTRO~OLITAN ARfA TRAfFIC STUDY 1955

1980 VEHICULAR DESIRE LINE DENSITIES - DIRECTION B 41

Mop 8.

672

681

•••

42

. ..,

692 693

1 ...

622 633

621

r 623

6 12

620

t

LEGEND

This chart shows the pattern of travel desire of 1,356, 141 inte r­zonal or station group vehicle trips in 1980 traveling in direc­tion "C".1 A total of 7,056,724 airline miles of vehicular travel ore traced on this chart. All desire lines are traced through a one mile grid system.

Desire line traces per

1 8y assuming that a l l trios beQin at the southernmost end of the t rip, tr ip desire lines are classified into one of four oossible direc t ions as shown below. Direction "C" in­cludes due northwest and uo to but not including due west directions.

D

• D D D

square

50,000

40,000

30,000

20,000

10,000

mile

to 54,000

to 49,999

to 39,999

to 29,999

to 19,999

OfTROIT MfTROPOLITAN ARfA TRAttiC STUDY 1955

DETROIT TRAFFIC STUDY AREA

TRIP DESIRE CHART

1980 VEHICULAR DESIRE LINE DENSITIES- DIRECTION C

Map 9.

r I

r,

•••

•••

693

DETROIT TRAFFIC STUDY AREA

TRIP DESJRE CHART

LEGEND

This 'hart shows the patte rn of travel d esire of 1,501 ,268 inter­zonal or station group vehicle trips in 1980 traveling in direc­tion " 0 ".1 A total of 8,457,748 airline miles of vehicular travel are traced on this chart. All · desire lines are traced through a one mile grid system.

Desire line traces per

18y assuminQ that a l l trios begin at the southernmost end of the tr ip, trip desire lines are classi f ied into one of four possible direct ions as shown below. Direction "D" in­c ludes due east and up to but not includinQ due northeast directions.

N

-" I. /'-' W~[

D D D D D

square mile

50,000 to 58,500

40,000 to 49,999

30,000 to 39,999

20,000 to 29,999

10,000 to 19,999

08ROIT M8RO~OLITAN ARfA TRAffiC £TUOY 1955

1980 VEH ICULAR DESIRE LINE DENSITIES - DIRECTION D 43

Map 10.

.. --:-.~.-..

. .

, ..

. ,,'

0 '

As in each preceding map, the Central Busi­ness District has a powerful effect in concentrat­ing travel desire. In this direction there are two possible approaches to the CBD. This map therefore has the greatest length of blue area of any of the four directions. The heaviest desire volumes near the Central Business Dis­trict produce the spot of greatest desire density well east of the center of town. This is caused, as in the other example, by the concentration of trip desire as the Central Business District is approached. In this case the fact that the blue area extends through the Central Business Dis­trict suggests that there is a substantial amount of through movement. If this were not the case, there would be a drop in desire line intensity at the Central Business District due to the fact that only half values are used for trips at the terminal grid point.

The darkest blue area results from a series of overlapping movements rather than from con­tinuous traveL A section of the blue results from travel to and from the River Rouge indus­trial area, whereas the second dark blue group of trips is going to and from downtown Detroit. The two parts can be identified by the change in direction of the section of blue area.

The isoline enclosing trip desire densities greater than 40,000 trips per square mile fol­lows the general location of Michigan A venue and East Jefferson. Such future desire densities will certainly create need for additional road capacity in these areas.

A small pattern of darkest blue points toward the New Center Area on the west side of Wood­ward again showing how this secondary center, a great traffic generator, can focus trip desires.

The dark yellow area shows a concentrated, continuous desire for service across the northern part of the City and suggests the need for ade­quate crosstown routes to meet the future travel demand.

This is the last of the four directional maps of 1980 vehicular trips. These four maps pro­vide much more detail on the magnitude, the

44

location and the direction of the numerous future traffic movements, than does the single, summary map. Likewise they further illustrate the reasonableness of the trip forecast. Con­stant reference will be made to these directional maps as optimum route locations for new expressways are considered in later sections of this report. 8

SUMMARY

In this chapter the method for predicting zone-to-zone movements for the year 1980 were described. A method was developed which con­sidered the amount of change in traffic genera­tion at every zone and the relative attractiveness of each possible movement between pairs of zones. When the complete forecast was made, the movements between zones added up to the number of predicted trips to and from each zone or station group. The results of the forecast of travel were detailed on desire line charts and compared to the patterns inventoried in 1953. The results of the 1980 forecast were tested and examined in other ways and were shown to be quite reasonable. The forecast showed that where population increased by 48 % , car owner­ship increased by 61%, vehicle trips by 67% and vehicle miles by 7 6%.

Finally, the 1980 vehicular trips were clas­sified by direction and four directional desire line density charts prepared. These directional charts provided a much more detailed view of the expected future traffic pattern and were shown to provide a helpful tool for determining the location of future improvements.

This effective simulation of future traffic pattern represents a great improvement in the technique of traffic forecasting . The techniques shown will provide one system for economically and effectively keeping this traffic inventory up to date. If this is done, the investment in data collection will not be so rapidly lost as has been the case with other surveys.

8For further study of these maps the reader is referred to comparable directional maps in Chapter IV, Part I prepared for 1953 vehicular travel.

• : I