E 311 Volume

The specific types of traffic volume counts that are generally used by the street designer are as follows.

E 311.1 Annual (Total)

Continuous traffic counts (or periodic adjusted counts) are taken of the total volume within a street without regard to direction of flow for a period of one year. This annual count in terms of vehicles per year is used in preparing traffic flow maps, determining future trends, and computing accident rates (per million miles).

E 311.2 Average Daily Traffic

Average Daily Traffic (ADT) in vehicles per day represents the total traffic for the year (Subsection E 311.1) divided by 365 days, or the average volume per day. This count is used in the developing of the developing of the arterial street system. The programming of capital improvements, and the design of structural elements. Its direct use in the geometric design of streets and highways is not appropriate because it does not indicate the significant variation in the traffic occurring during the various months of the year, days of the week, and hours of the day. 

Since traffic volumes may either greatly exceed or fall far below the ADT for the considerable number of years or days of the year, the capacity of a highway designed solely on the basis of the ADT count would be incapable of handling efficiently the greater volumes for those periods of very high volumes.

E 311.3 Hourly Traffic

The traffic pattern for most City streets shows considerable variation in traffic volume during different hours of the day (such as rush hour) and even greater volume variation throughout the year. The most satisfactory measurement of these fluctuations in traffic volume for design purposes is a period of one hour. However, the design should not be predicated on the maximum peak hour of the year because the traffic would rarely reach sufficient volume to make full use of the resulting facility. Yet, using the average hourly traffic as the design volume (which would be determined by averaging the maximum volume during peak hours with the minimum, say, after midnight) would produce a design totally inadequate to handle peak loads.

Various studies have been made to determine the range of the peak hourly volumes and the relationship of these peak hours to the ADT in terms of number of vehicles and percentages. For purposes of this Part of the Manual, all of the factors considered leading to the following conclusion need not be included in this manual. For a more comprehensive treatment, reference is made to various previously mentioned sources shown in Section E 020(F).

The conclusion reached by most authorities is that the 30th highest hourly volume for the year is the optimum choice from the standpoint of both economy and vehicular accommodation. This means that the peak hourly volume of traffic can be always accommodated except when it exceeds the 30th highest hourly volume. The de- sign hourly volume (DHV) should be the 30 HV of the future year chosen for design. Where streets have a high seasonal traffic fluctuation or where the traffic flow is radically different, the use of the 10th, S0th, 50th, or some other value may be more accurate.

The conclusion reached by most authorities is the 30th highest hourly volume for the year is the optimum choice from the standpoint of both economic and vehicular accommodation. This means that the peak hourly volume of traffic can be always accommodated except when it exceeds the 30th highest hourly volume. The design hourly volume (DHV) should be the 30 HV of the future year chosen for design. Where streets have a high seasonal traffic fluctuation or where the traffic flow is radically different, the use of the 10th, 20th, 50th, or some other value may be accurate.

DHV (in vehicles per hour) finds its greatest application in:

1. Determining the length and magnitude of peak periods.
2. Evaluating capacity deficiencies.
3. Establishing traffic controls, since volume is one of the warrants for the:
   1. Installation of signs, signals, and markings.
   2. Designation of through streets, one-way streets, unbalanced flow, and traffic routing.
   3. Prohibition of parking, stopping, and turning.
4. Geometric design or redesign of streets and intersections.

E 311.4 Directional Movements

The foregoing discussion relating to the ratio of the ADT to the 30th DH takes into account only the total traffic movement in both directions. On all multilane facilities, however, consideration of the directional traffic loads may be of greater importance.

For example, a tabulation of traffic by direction of movement shows that in downtown areas during peak hours, an average of 55 percent of the traffic was moving in the heavier direction, in intermediate areas 61.6 percent, and in outlying areas 65.6 percent.

As an illustration of the effect of the above, a four-lane highway in a rural area has a lane capacity of 1500 vehicles per hour and a nominal total capacity of 6000 vehicles per hour. During the rush hour the directional traffic (using the above value of 65.6 percent) will total 4500+ vehicles per hour. The highway capacity is therefore inadequate for carrying the peak load.

For this reason, in contemplating the design of a highway and in determining its capacity, the DHV for one direction should be computed by multiplying the total two-way ADT by the percentage that the 30 HV is of the two-way ADT and by the percentage of traffic in the predominant direction during the design hour. Thus, if the two-way design hourly volume is 15 percent of the two-way ADT and the directional distribution is 60-40 percent, the one-way DHV is 0.15 x 0.60 x ADT, or 9 percent of the two-way ADT.

E 311.5 Composition of Traffic

Vehicles of different sized and weights have different operating characteristics which must be considered in determining highway capacity. The larger the proportion of trucks in a traffic stream, the greater the effective traffic load, and the more highway capacity is needed. For example, Figure E 211.5 shows that with a percentage of 20 percent of trucks m a traffic stream, the capacity of the Highway is only 63 percent of that highway carrying only passenger vehicles. Although this chart indicates only the effect of truck traffic and terrain on capacity, there are numerous other factors that will further reduce capacity, such as bus stops, slower turning movements at intersections, etc. These last-mentioned factors are discussed under Section E 650, Intersections at Grade.

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Truck traffic is expressed as the percentage of total traffic during the design-hour, total two-way traffic in the case of a 2-lane highway, and total traffic in the predominant direction of travel in the case of multilane highways.

Since peak truck traffic does not usually coincide with peak passenger flow, observations must be made of truck traffic percentage which is considered representative of the 30th highest or de- sign hour.

311.6 Projection of Traffic

New highways or improvement of existing highways should be based not on current traffic volume alone, but on the future traffic expected to use the facility. Estimating of future traffic for a particular facility is generally based on a 20-year period. There is little justification for going beyond this point because of probable changes in the general regional economy, population trends, and land development abutting and in the vicinity of the highway.

Traffic can be estimated with reasonable accuracy where there are sufficient current and past traffic data available and where contemplated or likely development within the near future, that is apt to affect traffic flow, is known. Components used to determine future traffic are as follows:

1. Current Traffic
   1. Existing traffic.
   2. Attracted (or diverted) traffic.
2. Traffic Increase.
   1. Normal traffic growth.
   2. Generated traffic.
   3. Development traffic.

Expanding current traffic volumes to future volumes is done on the basis of daily volumes.  Design volumes, which are usually hourly volumes, are obtained by applying appropriate factors, as will be shown in the succeeding discussion.

E 311.61 Current Traffic, Existing and Attracted

Current traffic is the volume of traffic that would use a new or improved facility if it were open to traffic.  In the improvement of an existing facility, the current traffic is made up of the existing traffic using the highway (prior to improvement) plus the existing traffic attracted to it from other facilities upon completion. In the case of a new highway, the current traffic is made up entirely of attracted traffic.

Depending on the type and location of the highway, current traffic may be established from:

1. Traffic counts on existing facilities that are apt to influence the flow of traffic on the improvement (the traffic counts are the existing ADT on the highway plus the ADT attracted to it from other facilities).
2. Roadside interview origin-and-destination studies.
3. Comprehensive (home interview) origin-and destination studies in suburban or urban areas.
4. A combination of these counts or studies.

E 311.62 Traffic Increase

After the current traffic volume on an improvement is established, it is then necessary to determine the probable traffic in some future year selected for design The current traffic, which represents only the existing trips that would be made over the improvement when opened to traffic, will be increased by normal growth, generated traffic, and development traffic.

E 311.621 Normal Traffic Growth

Normal traffic growth is the increase in traffic volume due to general increase in number and usage of motor vehicles.

By the exercise of good judgment and in consideration of the increase in population and in motor vehicle registration and travel, a curve of the past trend of motor vehicle increase per year can be projected. This projection will show the estimate and trend for future vehicle increase and use at the region, state, or locality in a particular highway is situated.

As an example, assume that from the values picked off the projection, the current year shows an increase of 7.5 percent and 11.2 percent for the future 20 years hence. The current ADT volume is 6000 vehicles. The future volume 20 years hence, including normal traffic growth, can be determined by the following calculation:

and 6000 vehicles x 1.49%= 8940 vehicles (20 years hence).  Values for generated and development traffic must be determined separately and added to this value.

E 311.622 Generated Traffic

Generated traffic consists of motor vehicle trips (other than by public transit) that would not have been made if the new facility had not been provided. Generated traffic made up of three categories:

1. New trips not previously made by any mode of travel.
2. Trips that previously were made by public transit.
3. Trips that previously were made to a different destination, but for which the change is attributable to the attractiveness of the improved high- way and not to change in land use.

Regardless of the reason for generated traffic, the net result is traffic increase on a given improvement. Most of this traffic develops within the first year or two after a new facility is opened.

In areas where data are not available, generated traffic will have to be based almost entirely on judgment, with experience in other localities serving as a guide. Logical estimates may include generated traffic as being generally 5 and 25 percent of the current traffic.

E 311.628 Development Traffic

Traffic due to improvements on adjacent land over and above the development which would have taken place had not the new or improved highway been constructed is called “development traffic.” This component of future traffic, unlike that of generated traffic, continues to develop for many years after a new facility is constructed: Increased traffic due to normal development of adjacent land is included in normal growth, but experience with many improved highways indicates that adjacent land is developed more rapidly than land elsewhere. The additional traffic resulting therefrom should be accounted for in estimating future traffic volume.

For example, it is more likely that vacant land will be built upon than that additional improvements will be made on land already occupied with buildings. Land near railroads and watercourses is likely to encourage industrial development, and high ground is likely to be improved with homes. Past trends and the opinions of local businessmen are valuable guides in predicting future development.

In general, however, the method of obtaining development traffic appears crude and the volumes obtained are only approximate, but in many cases the volume from this cause is appreciable. The omission of this item in estimating future traffic may explain why the volumes for a number of prominent improvements in the past proved to be underestimated. 

E 311.624 Traffic Projection Factor

The ratio of future traffic to current traffic is called the “traffic projection factor.” The traffic increases that this factor reflects combine those due to normal traffic growth, generated traffic, and development traffic, previously discussed.  The future year (for design) should be specified with every traffic projection factor.  The traffic projection factor is then obtained by adding the percentages of increase for each item of traffic growth in relation to current traffic, dividing the sum by 100, and adding 1 to the result.

For example, the current traffic count (present- year) on an existing street in which further improvement is contemplated has an ADT of 24,000 vehicles. The design year is 20 years hence. Based on the previous discussion, the estimated normal traffic growth 20 years hence is 68% of the present year traffic.  Generated traffic is assumed to be 18% of the current traffic.  Development traffic is estimated to be 8200 trips per day on this street section 20 years hence, or an increase of 8200/ 24,000=34% of the current volume.  Thus, the total traffic increase is 68 + 18 + 34 = 120 percent. The traffic projection factor 20 years hence is 2.20. and future ADT = 24,000 x 2.20 = 52,800 vehicles.

E 311.625 Design Traffic Data

The design hour volume represents the “load” which the street or highway must accommodate and determines, to a large degree, the type of facility, pavement widths, and other geometric features. The usual procedure employed in arriving at the DHV where the essential design traffic data are available may be demonstrated as follows: 

It was found in the example given in Subsection E 311.624 that an ADT of 52,800 vehicles will use the improvement in the design year 20 years hence. Local studies indicate that the DHV is 13% of ADT, or K = 13%, the two- way directional flow (D) = 61% , and the truck traffic (T) is equal to 7% of the one-way DHV. Using these data, the DHV 20 years hence = 52,800 x 0.13 x 0.61 4200 vehicles in one direction of travel, of which 7% are trucks, or approximately 3900 passenger cars and 300 trucks.