§ 3.4.13. Traffic Data

Traffic data is used by both the Municipal Flexible Pavement Design and Municipal Rigid Pavement Design programs to generate the pavement structural designs and to calculate the user costs associated with delays during overlay construction. Thus, this data is significant and worthy of considerable attention.

The ADT, (initial ADT) on an existing street may be taken from current traffic counts (within past 12 months) if the proposed street character is not changed. Major widening of an existing street would require a more comprehensive analysis. When a street is cut through a new area, the values from Table 3-11 should be used, except when a Traffic Impact Analysis (TIA) or a Traffic Assessment (TA) is available which includes an estimate for ADTo, these values should be used.

A.

Average Daily Traffic

Traffic growth within the design models is assumed to be compounded yearly. Thus, the accumulation of traffic loadings is exponential. The growth rate of average daily traffic is the rate (expressed as a percent) at which traffic grows each year. The City has generally experienced the following growth rates:

	Percent Per Year
Residential	3
Residential Collector	3.5
All other streets	4

 

The following equation shows how the growth rate, g, expressed in percent, is used in the programs to estimate a future average daily traffic in year n for estimating traffic delay costs given the initial traffic, ADTo.

B.

Directional Distribution Factor

This factor is used to quantify a difference in the overall direction of load movement. Generally, this value is 50% since the number of vehicles going in averages those going out. There may be cases such as a factory, however, where trucks enter heavily loaded with raw materials and leave lightly loaded with the finished product. Since heavier loads are much more damaging than lighter loads, there may be justification for the use of a directional distribution factor greater than 50% for the design of the heavier lanes.

C.

Lane Distribution Factor

This factor accounts for the lateral distribution of truck traffic. For two lane facilities, this factor is 100% since all truck traffic (in one direction) must use the same lane. According to the American Association of State Highway and Transportation Officials Interim Guide for Design of Pavement Structures (1972), the distribution for four lane facilities is between 80% and 100%. For six or more lanes, the distribution is between 60% and 80%. Values from the American Association of State Highway and Transportation Officials Design Guide of 100, 90 and 80, respectively, are recommended for two, four and six lane facilities.

D.

Percent Trucks In Initial Average Daily Traffic

This is one of the factors used to convert traffic (of all vehicle types) into the 18-kip equivalent single axle loadings used for pavement structural design. Higher truck percentages will result in overall thicker pavement structures which have increased load-carrying capacity.

E.

18-kip Equivalency Factor for Average City Truck

This factor is used to convert truck and bus traffic into the 18-kip equivalent single load applications used to design the pavement structure. Although the description of this variable implies that it is a general factor which can be applied on a city-wide basis, it is not. It is dependent upon the type of pavement structure (flexible or rigid), the types of trucks which use the facility, their distribution and their loading. A small study was conducted in order to establish some preliminary criteria for use in this manual.

Texas State Department of Highways and Public Transportation loadometer data was used to define the different equivalency factors for different class vehicles on the two pavement types. Traffic and truck counts from seven Austin city streets were used to determine a weighted 18kip equivalency factor for the "average" city truck on a given class facility. The results in terms of recommended 18-kip equivalency factors are presented in Table 3-8. Note that a factor of safety was used in establishing these recommendations. This factor of safety is intended to reduce the risk associated with the uncertainty in the equivalency factor estimations.

TABLE 3-8 18-KIP EQUIVALENCY FACTOR RECOMMENDATIONS (FOR AVERAGE CITY TRUCK) AS A FUNCTION OF STREET CLASSIFICATION
	Recommended 18-kip Equivalency Factors for Average City Truck
Street Classification	Flexible	Rigid
Residential (FS = 1.2)	0.4	0.46
Neighborhood Collector (FS = 1.2)	0.53	0.58
Minor Arterial (FS = 1.4)	0.62	0.66
Major Arterial (FS = 1.4)	0.84	0.97
Note: FS refers to the "built-in" factor of safety in each equivalency factor recommendation.
Source: ARE, Inc.

 

Should more truck counts and traffic data be collected by the City, the 18-kip equivalency factors in Table 3-8 should be revised using a weighted averaging procedure and estimated urban truck equivalency factors obtained for the Texas State Department of Highways and Public Transportation.

F.

Initial Average Daily Traffic

This variable represents the projected two directional average daily traffic during the first year after construction of the facility. It is used to estimate both design 18-kip ESAL traffic and user delay costs.

Note: If the user wishes to determine the total cumulative 18-kip equivalent single axle load traffic over the analysis period (that will be used in the pavement structural design), the following equation may be used:

Where:

n = length of analysis period (years)

W _n = cumulative 18-kip equivalent single axle traffic during analysis period in the design lane

g = traffic growth rate expressed as a percent

Wo = is the initial 18-kip equivalent single axle traffic and is determined using this equation:

W _o = ADT _o × TP × LDF × DDF × TEF × 365 × 10 ^-6 (Eq. 3-6)

Where:

ADT _o = initial average daily traffic (two directions)

TP = truck percentage

LDF = lane distribution factor (percent)

DDF = directional distribution factor (percent)

TEF = average 18-kip equivalency factor for a truck on the design facility