APPENDIX
DRAFT TRAFFIC FORECAST
INTRODUCTION
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The Logan Canyon Environmental Study requires an evaluation of present and future traffic needs. Present needs can be identified using existing information on roadway design and current traffic volumes. However, in order to assess future traffic needs, an estimate of future traffic volumes is needed. This technical memorandum describes the present traffic patterns in Logan Canyon (Canyon), the techniques used to forecast future traffic volumes, and the Average Daily Traffic (ADT) volume forecast for the year 2010. After the ADT has been determined, an appropriate peaking factor (based upon recorded peak hour traffic volumes) will be selected. The design traffic volume will then be established using the peaking factor (the peaking factor is the percent of the ADT which occurs during the peak hour) .
AVAILABLE DATA
Data on current and past traffic volumes in Logan Canyon and roads in the surrounding area is available and has been used in the preparation of this memorandum. The information available is as follows:
o Permanent counting station on US 89 at the Card Guard Station in Logan Canyon, from 1973 to 1983.
- Daily and hourly directional volumes
- Monthly and annual summaries
o Permanent counting station on US 89 west of Garden City, from 1983 to present (moved from Card Guard Station) .
- Daily and hourly Directional volumes
- Monthly and annual summaries
o Permanent counting station on US 89 north of Garden City from 1966 to present.
-Daily and hourly directional volumes
- Monthly and annual summaries
o Permanent counting station S-89-91 on Sardine Summit, from 1975 to present.
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- Daily and hourly directional volumes
- Monthly and annual summaries
o Permanent counting station us 91 at Webster Junction, north of Logan, from 1966 to present.
- Daily and hourly directional volumes
- Monthly and annual summaries
o Manual counts taken during 1984, 1985 and 1986.
o "1986 Baseline Projections", State of Utah, Office of Planning and Budget.
The two permanent counting stations on US 89 and information presented in 1986 Baseline Projections were used as primary sources of data in the preparation of forecasts. The manual counts were used to provide classification data and distribution of traffic through the Canyon. The permanent stations on US 91 were used for reference only.
CURRENT TRAFFIC VOLUMES
The past traffic volumes in the Canyon have been summarized in Table 1. Traffic volume in the Canyon is highly seasonal. Summer ADT's are often as much as three times greater than winter ADT's and nearly twice the annual ADT. The increase in summer traffic volume is attributable to the recreational opportunities in the Canyon, the resort and condominium development around Bear Lake, and through traffic to Yellowstone and Teton National Parks. Because of the highly seasonal variation in traffic volumes in the Canyon, it is recommended that the summer traffic volumes be used to prepare a forecast ADT. The annual summaries taken from data gathered at the permanent counting stations, and the results of the manual counts taken to date, have been included as an appendices to this memorandum.
FORECASTING TECHNIQUES
Traffic forecasting is not an exact science. The function of our roads and highway is primarily to provide a safe, and where possible convenient, means of passage from one point to another. It is for this reason that when forecasting traffic volumes, it is necessary to look at the factors that would cause a change in traffic volume. Economic and population growth will usually bring about increased traffic volumes as will changes in land use.
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The "Transportation and Traffic Engineering Handbook", published by the Institute of Traffic Engineers discusses two techniques used to forecast future traffic volume. One technique uses expected changes in population and economic conditions to estimate future traffic flows, and the other uses anticipated changes in land use. However, both of these techniques are discussed in relation to urban and regional or statewide planning. The State of Utah also utilizes a method of forecasting traffic volumes which utilizes anticipated changes in land use and population in an urban area. Cache and Rich Counties are not urban population centers and these techniques will have to be carefully applied if they are to be used and provide satisfactory results on US 89 through Logan Canyon.
For this study, three techniques for forecasting have been utilized and the results of each compared before recommending a planning level ADT. These three techniques are:
o Projection of past trends
o Economic and population forecasts
o Landuse change forecasts
TRAFFIC FORECASTS
PAST TREND FORECASTING
A past trends forecast assumes that the trend established by past changes in traffic volume will continue into the future. If the period of record is long enough, the impact of changing economic and population conditions should be reflected in the trend. For this study, two types of analysis have been utilized in preparing a forecast using this technique. A "least squares best fit" line has been determined for the existing data and the line extrapolated into the future . The least squares approach assumes a linear increase in traffic volume, which is not usually the case. Typically, long term growth will approximate an exponential curve, so for this study, an exponential function was also fit to the data. The increases in traffic volume forecast using the best fit and exponential analysis are summarized in Table 2.
This type of analysis has two weaknesses when applied to Logan Canyon.
1) The period of record is short, especially when compared to the length of the planning period. In this case the function is extrapolated over a
long period of time, 2.5 times the period of record.
2) There is also a significant scatter to the data points. With so few points and significant scatter, it is difficult to feel that the forecast is as reliable as would be prefered.
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In an effort to overcome these weaknesses a similar forecast was prepared for the station on US 89 north of Garden City This station has a longer period of record and may provide an indication of the long term trend in the general area. The results of those analysis are summaried in Table 3.
Upon comparing the forecast increase at each station, the summer traffic using an exponential function to fit data points, shows a total increase of 75 percent from 1980 to 2010 in the Canyon and a total increase of 77 percent on US 89 north of Garden City. The straight line extrapotalions show a larger spread (45 and 52 percent). Figures 1 and 2 graphically illustrates these past trends based forecasts.
ECONOMIC AND POPULATION FORECASTING
Using economic and population forecasts as a basis for estimating future traffic volumes assumes a relationship between population growth, economic activity, and changes in traffic volume. In preparing economic and population forecasts, specialists in economics, geography, demography and other social sciences all combine their expertise to develop projections of population growth and economic activity. Estimates of future changes in traffic volumes are then made based upon forecast changes in population and economic conditions.
The State of Utah, Office of Planning and Budget prepares an annual projection of Utah's population and economic conditions.
The "1986 Baseline Projections" which project population and economic conditions through the year 2010 has recently been released. In the report it states that it is, " ... an annual update of the baseline or most likely economic and demographic conditions, through the year 2010, for the State of Utah, its counties, and its multi-county planning districts ... ". Table 4 summarizes the expected populations within the Bear River and Wasatch Front areas. The area is expected to increase in population from approximately 1,156,150 in 1985 to 1,860,500 in 2010; an average compounded increase of approximately 1.95 percent per year.
Using a projected annual growth rate of 1.95 percent, the ADT in Logan Canyon at the Card Guard Station would be expected to increase from 3276 vehicles per day in 1980 to 5847 vehicles per day in 2010. This compares closely with the 5721 vehicles per day forecast using the past trends techniques. The increase in traffic from 1985 to 2010, using economic and population change as a basis for traffic forecasts, is summarized .in Table 5. Figure 3 is a graphical illustration of the past trends and economic and population ADT forecast.
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LAND USE FORECASTING
Using land use as a basis for projecting future traffic volume assumes there will be a significant change in land use. Most of US 89 in Logan Canyon is in the Wasatch-Cache National Forest. The Forest has recently completed it's Forest Management Plan, and no significant changes in management of the land in and adjacent to Logan Canyon is planned. Therefore, there is no reason to believe that there would be an increase in traffic as a result of actions taken by the Forest Service.
There is much private land in Rich Country adjacent to Bear Lake and nearby. At the present time, this private land is primarily being used for agriculture. The lower land being cultivated with the higher and timbered land being used for livestock grazing. There is much interest however in developing recreational facilities around Bear Lake. A number of cabin and condominium developments have been built and many more are planned. In 1981, a survey showed 1011 developed cabins or condominium units along the south and west shores. At that time, there were also 17,420 planned units, an increase of over 1,600 percent. If all of these units were to be completed by the year 2010 (as the developers hope), the summer ADT would increase to 53,171 vehicles per day at the end of the planning period. While continued recreational
development is to be expected around Bear Lake, it is unlikely that it will have the impact on Logan Canyon traffic shown using this technique.
CONCLUSIONS AND RECOMMENDATIONS
The past trend and population and economic growth techniques for forecasting future traffic volumes produced a surprisingly close estimate of the year 2010 summer ADT (comparing the exponential function past trends forecast with the population and economic growth forecast). A forecast based upon anticipated recreational development around Bear Lake produces a volume over nine times as great as the other two
techniques. It is not unreasonable to assume that the increase in traffic volume in Logan Canyon would generally follow the increase in population in northern Utah. It is therefore recommended that the ADT for the year 2010 used in this study be 5900 vehicles per day.
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