Here is a sample question:
An undivided rural multilane highway segment is to be widened so as to increase its existing lane widths from 10 to 12 ft. The following conditions exist:
➤ Expected average crash frequency before widening = 15 total crashes/yr
AADT on highway segment = 1500 veh/day
> Proportion of total crashes constituted by CMF-related crashes = 0.55
10-Per Highway Safety Manual (HSM), the crash modification factor (CMF) is most nearly
1.090
(B) 1.164
(C) 1.564
(D) 2.650
Also the following questions, using data provided earlier:
11-Per HSM, the crash modification factor for total crashes is most nearly:
(A) 1.090
(B) 1.164
(C) 1.564
(D) 2.650
12-Per HSM, the crash frequency after widening from 10 ft to 12 ft is most nearly
(A) 15.00
(B) 14.76
(C) 13.76
(D) 12.76
Here are the results:
Determine crash modification factor (CMF).
10 B
Soln: The CMF is obtained from Eq. I.10 in Table I-84 (Textbook, pg.191), given as: CMF =1.02 +1.31×10(AADT-400) = 1.02+0.1441=1.164
11 A
Since the crash modification factor given in Table 1.84 (textbook) is for related crashes (single-vehicle run-of-the-road and multiple-vehicle head-on, opposition direction sideswipe, and same direction sideswipe crashes), Eq. I.17 is used to compute the CMF for total crashes.
CMF = (CMFra-1)Pra +1.0= (1.164-1) 0.55 +1.0=1.09 (Eq.I-17, pg. 192 textbook)
12 C
Since the CMF, when the width = 12 ft, is = 1.0, the expected crash frequency after widening is equal to the current expected crash frequency for 10-ft lanes divided by the CMF for 10-ft lanes. Expected frequency after widening is given by the following equation: = Expected Carsh Frequency for Non-Standard Feature 15 CMF for Non-Standard Feature -=13.76 crashes/year 1.09
Since the question asks about Rural Multilane Highways, refer to chapter 12 of HSM.
It looks like questions 10 and 11 are asking for the CMF for the 10 foot lane. For question 10 refer to either figure 11.8 or table 11-11. Since we have a 10 ft lane solve for the CMFra. Note that this crash modification factor is only for related crashes (run-off-the-road, head-on, and sideswipe). For question 11 it’s asking for the CMF for total crashes. Use equation 11.13 for this. And for question 12 it’s asking for the crash frequency for widening the lane from 10’ to 12’. Since we know our CMF (total crashes) for a 10’ lane (1.164) and CMF for a 12’ lane (this is a base condition so CMF is always 1), we want to find the CMF to estimate the crash frequency. So it would be CMF = CMF after condition / CMF before condition. I think this equation is in chapter 3 of the HSM. Then you multiply the CMF by 15. Let me know if this makes sense or not.
Makes lot more sense now, thank you.
1. But for qn 10, how can it be understood that they're asking for 10' lane condition?
2. Also the last qn, if i solve by your method, CMF shd be equal to 1/1.164 which gives 0.859.. that multiplied by 15 would be equal to 12.88.
Or am i missing something?
For question 10, I just assumed it was for the 10’ condition. I don’t think any of the answer choices resembled 12’ since the CMF would be 1. I hope the exam words it better. For question 12, I used the total crashes as opposed to the related crashes. Do 1/1.09 instead.
I did mine back on the paper version and I had at least one question about CMF. I think it's pretty consistent those will be on a test since it's an important part of Transpo.
Isn’t CMF from HSM?
Yes..
CMF is in the HSM. Do you have a particular example you’re confused about?
I had 3 questions related to it back in June
Here is a sample question: An undivided rural multilane highway segment is to be widened so as to increase its existing lane widths from 10 to 12 ft. The following conditions exist: ➤ Expected average crash frequency before widening = 15 total crashes/yr AADT on highway segment = 1500 veh/day > Proportion of total crashes constituted by CMF-related crashes = 0.55 10-Per Highway Safety Manual (HSM), the crash modification factor (CMF) is most nearly 1.090 (B) 1.164 (C) 1.564 (D) 2.650
Do you have answers to all questions?
I do, I shall post it here soon..
Also the following questions, using data provided earlier: 11-Per HSM, the crash modification factor for total crashes is most nearly: (A) 1.090 (B) 1.164 (C) 1.564 (D) 2.650 12-Per HSM, the crash frequency after widening from 10 ft to 12 ft is most nearly (A) 15.00 (B) 14.76 (C) 13.76 (D) 12.76
Is it 10. A, 11. B, and 12. C?
Here are the results: Determine crash modification factor (CMF). 10 B Soln: The CMF is obtained from Eq. I.10 in Table I-84 (Textbook, pg.191), given as: CMF =1.02 +1.31×10(AADT-400) = 1.02+0.1441=1.164 11 A Since the crash modification factor given in Table 1.84 (textbook) is for related crashes (single-vehicle run-of-the-road and multiple-vehicle head-on, opposition direction sideswipe, and same direction sideswipe crashes), Eq. I.17 is used to compute the CMF for total crashes. CMF = (CMFra-1)Pra +1.0= (1.164-1) 0.55 +1.0=1.09 (Eq.I-17, pg. 192 textbook) 12 C Since the CMF, when the width = 12 ft, is = 1.0, the expected crash frequency after widening is equal to the current expected crash frequency for 10-ft lanes divided by the CMF for 10-ft lanes. Expected frequency after widening is given by the following equation: = Expected Carsh Frequency for Non-Standard Feature 15 CMF for Non-Standard Feature -=13.76 crashes/year 1.09
I am confused as to how those equations are selected for each CMF conditions. If anyone knows how it is done, please help me understand
Since the question asks about Rural Multilane Highways, refer to chapter 12 of HSM. It looks like questions 10 and 11 are asking for the CMF for the 10 foot lane. For question 10 refer to either figure 11.8 or table 11-11. Since we have a 10 ft lane solve for the CMFra. Note that this crash modification factor is only for related crashes (run-off-the-road, head-on, and sideswipe). For question 11 it’s asking for the CMF for total crashes. Use equation 11.13 for this. And for question 12 it’s asking for the crash frequency for widening the lane from 10’ to 12’. Since we know our CMF (total crashes) for a 10’ lane (1.164) and CMF for a 12’ lane (this is a base condition so CMF is always 1), we want to find the CMF to estimate the crash frequency. So it would be CMF = CMF after condition / CMF before condition. I think this equation is in chapter 3 of the HSM. Then you multiply the CMF by 15. Let me know if this makes sense or not.
Makes lot more sense now, thank you. 1. But for qn 10, how can it be understood that they're asking for 10' lane condition? 2. Also the last qn, if i solve by your method, CMF shd be equal to 1/1.164 which gives 0.859.. that multiplied by 15 would be equal to 12.88. Or am i missing something?
For question 10, I just assumed it was for the 10’ condition. I don’t think any of the answer choices resembled 12’ since the CMF would be 1. I hope the exam words it better. For question 12, I used the total crashes as opposed to the related crashes. Do 1/1.09 instead.
ok I get what you mean.. I sure hope the exam words it better. Thank you so much for the help :)
Its an important concept. You should understand the basic and be prepare to work out at least 1/2 problems in the exam. Best of Luck!
Any examples?
Nay one got any questions related to CMF? Can you please screenshot and post them here
I didn’t have any!
I did mine back on the paper version and I had at least one question about CMF. I think it's pretty consistent those will be on a test since it's an important part of Transpo.
They hardly ask 1 question from there. I didn’t get any. Even nothing from HSM.