The intersection delays generally will be calculated within TRAFFIX™ using the HCM Operations method. For those situations where delays have been calculated separately using field data or another level of service program, delays can be input in the link data dialog box. Where a full data set has been input on node screens to calculate intersection level of service, the results will be automatically included in the arterial analysis, even if they have not been selected for analysis themselves. It is possible for delays to be calculated, even if there is limited data available. The minimum data requirements are the following: (1) lanes and volumes in the direction of the route so that X (v/c ratio) can be calculated, and (2) an estimate for g/C. TRAFFIX™ will use the multi-lane factor as well as any other factors, such as PHF’s or PCE’s, that are appropriate either at the local or global level. For a planning analysis, the user may wish to turn off calculation of saturation flow adjustment factors, even though the defaults have been set, to minimize reductions.
The running time per mile for a given segment is based on three variables—arterial class, free flow speed, and average segment length. Arterial class itself is related to the functional category (principal or minor arterial) and its design category (suburban, intermediate, or urban). The HCM provides general criteria that can be used in selecting the design and functional categories. Table 16 includes four groups of values that have been left blank in the HCM Table. A brief explanation for each of these sections is presented below and in the footnote at the bottom of the table:
Area A: (Last row in table). These segments are long enough (greater than one mile in length) that deceleration and acceleration between intersections can be ignored. The running time for segments in this part of the table is calculated by dividing the number of seconds in an hour (3600) by the free flow speed in miles per hour.
Area B: (Upper left shaded area). For very short segments, the running time can be estimated from the value given in the table for the shortest segment. The value selected ranges from a 0.20 mile length for Class I arterials to a 0.10 mile length for Classes II and III. The HCM suggests that the arterial class be re-evaluated; TRAFFIX™ will perform the calculation but will generate an appropriate error message.
Area C: (Lower right shaded area). For lengths greater than 0.25 miles on Class II and III arterials, appropriate running time values have been estimated based on other values in the table. The HCM suggests that the arterial class should be re-evaluated if values in this area are used.
Area D: (First column on left). Free flow speed of 50 miles per hour. The HCM table does not contain any speeds greater than 45 miles per hour.
Table 16 Expanded Version of Table from the HCM
|
Segment Running Time per Mile for Use in Calculating Arterial Level of Service |
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|
Arterial Class |
I |
II |
III |
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|
Free Flow Speed (mph) |
50 |
45 |
40 |
35 |
35 |
30 |
35 |
30 |
25 |
|
Average Segment Length (mi.) |
Running Time per Mile (seconds/mile)1 |
||||||||
|
0.05 |
105 |
109 |
115 |
125 |
145 |
155 |
165 |
227 |
265 |
|
0.10 |
105 |
109 |
115 |
125 |
145 |
155 |
165 |
180 |
220 |
|
0.15 |
105 |
109 |
115 |
125 |
135 |
141 |
140 |
150 |
180 |
|
0.20 |
105 |
109 |
115 |
125 |
128 |
134 |
130 |
140 |
165 |
|
0.25 |
100 |
104 |
110 |
119 |
120 |
127 |
122 |
132 |
153 |
|
0.30 |
96 |
99 |
102 |
110 |
110 |
123 |
111 |
126 |
144 |
|
0.40 |
91 |
94 |
96 |
105 |
105 |
120 |
105 |
120 |
144 |
|
0.50 |
82 |
88 |
93 |
103 |
103 |
120 |
103 |
120 |
144 |
|
1.00 |
72 |
80 |
90 |
103 |
103 |
120 |
103 |
120 |
144 |
|
+1.00 |
72 |
80 |
90 |
103 |
103 |
120 |
103 |
120 |
144 |
|
Source: Table 11-4, 1994 Highway Capacity Manual and Dowling Associates. |
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