Star Sign up now to gain access to all the features of this website. NEW — access is now open to ALL USERS! — Already registered? Please log in.

Corrections and questions

Maria Alice Jacques
Independent Consultant
Corrections and questions was posted on September 17th, 2013 at 3:37 PM EDT
regarding Chapter 15: Two-Lane Highways

Is it possible to give some reference values for typical PHFs for two-lane highways as presented in Chapter 11 (page 11-13) and Chapter 14 (page 14-13) regarding freeways and multilane highways, respectively? In the examples presented in Chapter 15, PHFs range from 0.85 to 0.95.

Page 15-25: The percent time-spent-following calculated by Equation 15-9 is expressed in (%) and not (decimal). The value of BPTSFd, calculated by Equation 15-10 is multiplied by 100, and fnp,PTSF from Exhibit 15-21 is also expressed in percentage.

Page 15-31: The percent time-spent-following calculated by Equation 15-15 is expressed in (%) and not (decimal).

Page 15-28: The explanation related to the use of Equations 15-12 and 15-13 is not clear because of the constraint that the sum of the capacity for both directions has to be less than or equal to 3200 pc/h. The text says that when the directional distribution is other than 50/50, the base capacity is restricted to 1,700 pc/h in the direction with the heaviest flow, and capacity in the opposing direction is found by using the opposing proportion of flow, with an upper limit of 1,500 pc/h. Please look at the examples in the table below, and verify if they are correct or not. If they are not corrected, please indicate the corrected values to be adopted for the capacity calculations.

Directional Distribution Base capacity (pc/h) per direction Total base capacity

                                  Heaviest flow           Lowest flow   

50/50 1,600 1,600 3,200 51/49() 0.51x3,200 = 1,632 0,49 x 3,200 = 1,568 3,200 53/47() 0.53x3,200 =1,696 0,47x3,200 =1,504 3,200 55/45 0.55x3,200 =1,760 0,45X3,200 =1,440 3,140

                            than consider 1,700 

60/40 0.60x3,200=1,920 0,40x3,200 =1,280 2,980

                            than consider1,700  

(*) In these cases, the base capacity for the heaviest flow is set to 1,700 pc/h and for the lowest flow it is set to 1,500 pc/h?

Page 15-33: Step 5 – It would be helpful for the readers if the text informed that LOS for Class I highway is the worse of those calculated based on PTSF and ATS. This situation is only made clear in Step 5 of Example Problem 4 (page 15-55).

Page 15-35: Why in Equations 15-21 and 15-22 the indexes adopted for VMT and TT (index “i”) is different from those used in Equations 15-19 and 15-20 for VMT and TT (index “i15”)?

Page 15-46: In Exhibit 15-32, vo is expressed in (veh/h) and it should be expressed in (pc/h) as per Exhibit 15-15. Actually, the opposing demand flow, calculated in Step 3, is of 902 pc/h (and not 902 veh/h as stated in Step 4, at the bottom of page 15-45).

Page 15-54: Step 2 - The use of Exhibit 15-23 requires that vd is expressed in (pc/h). Therefore, for ATS vd = 902 pc/h, and for PTSF vd = 842 pc/h.

Sign in to add a reply. Don't have an account? No problem, sign up for free.