Cover Image

Not for Sale



View/Hide Left Panel
Click for next page ( 454


The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 453
required for system M1'BF and availability estimates. Figure A.4.2-1 illustrates Me concept wad a simple reliability block diagram. Tables A.4.2-la and A.4.2-Ib are spread sheets iBustradng He calculations. ~divt~ual paraDe! element failures must still be estimated to establish spares inventory and repair personnel requirements. A.4.3 Maintenance Personnel Staffing Expanding on discussions presented in Reference I,( rrE's 'traffic Signal Stallion and Maintenance Manual," Prentice HaB, 1989), maintenance aches typically involve he ac~vides discussed in Table A.4.3-~. Table A.4.3~1 Types of Maintenance Tasks Tvse Maintenance Description Comment . . Preventive Routine scheduled Usually follows equipment maintenance procedures manufacturers recommended task list, time intervals, and time requirements. Response/trouble Unscheduled maintenance to Variable work load that can correct a failure or mal- usually be reliably estimated function based on MTBF and MTTR. Design modification System enhancements that Staff loading to accommodate are accomplished by the \ system enhancements. Staffing maintenance staff. Is a function of requirements. ll it's maintenance manual outlines methods to estimate maintenance staffing levels for signal systems based on experience with fielded systems. This discussion win review ITS's methods and subsequent discussions win incorporate He concepts into life cycle cost estimating methods for ITS communication systems employing modem technology. Available maintenance staff yearly hours per person are calculated as follows: ~sNC~Phasez~p ~NCHRP3-51 Phase2F'nalReport A4-17

OCR for page 453
l co a' ~ \ ~ ~ ._ cn \ ._ C)~o in ~ ,- r) al u' 0 r o ~ o ~ ~ r) ~ ~ ~ ~ o ~ o a) A Q ~ m ~ Q I Lo ~ \ ._ _ o~ Q ~ ' ~ _.- - = a' U) ' ~ act, O ~ ~ Emus' 0 a' 11 11 11 11 llJ ~ 1 ` m ~ ~ ~ ~ :50 U] ~ L~ 1 D ~ a; ._ ~ ~n ~ a,_._\ '-' ~ ~ U) :~ Q ~ LO ~ Q O o O o Ln ~ CN O O CN ~@~\ .-~ cn ~n  ~ ~ C'`l O. O O ~ o ~ ~ L ~11 11 11 11 1 11 11 11 11 m ~ ~ Q = - ~ ~, _ ~ \ \ ~ ~ Q ~Q O O O L~ O ~ O 11 11 11 11 m ~ C' o~ 1 ~r m ~ ~ 11 ~ ~ . cn ao ~ ~ \ _ _ m~ ~ ~ .= \ '` \ Q.- = - ~ ~ (L) ' Q O a' OoLO ~ 0CNI 11 11 11 11 11 m ~ L~ ~ ~ OL ~ O 1~ _ N '~ ~ - 1 I ~ 1- ~ 111 ;) E~ \ o m~ 111 I 0, ~ .~ 11 11 11 11 a, J X ~ ~ ir ~ m ~ ao ~ _ _ - ~ ~ 11 ~ ~

OCR for page 453
Table A.4.2~1a Example Simple Reliability Network Excel Spreadsheet Equations Element A Element B Composite Upper MTBF(hr/fail) 100.0000 200.0000 66.6667 F(fail/hr) R(repair/hr) Ml1R(hr/repair) MTBF(hr/fail) F(fail/hr) R(repair/hr) MTTR(hr/repair) YRSHRS04.E96 0.0100 0.0050 2.0000 0.5000 0.5000 2.0000 Element A Element B 150.0000 125.0000 0.0067 0.0080 0.5000 2.0000 1.0000 1.0000 0.0150 1.5000 0.6667 Composite Lower 68.1818 0.0147 0.7727 1.2941 Parallel Composite Failure MTBF(hr/fail) 2318.18 F(fail/hr) 0.000431 R(repair/hr) 2.2727 MllR(hr/repair) 0.4400

OCR for page 453
- y- u^) ll ll c, ll - i: ~ Jo In o - or s - x LLJ it - . . lo: N t`) et Q _ ~ as x th i ~1~1~ ~J>~gbD.IP.~.~. 51 ~: e 5 ~5 ILLS 1 x to

OCR for page 453
Work hours per year Vacation: 3 weeks Sick Leave: 2 weeks Annual training: ~ week 2080 hours = 52 weeks x 40 hours/week 120 hours = 3 weeks x 40 hours/weak 80 hours = 2 weeks x 40 hours/week 40 hours = ~ week x 40 hours/week LegadHolidaysI3 days 104 hours =13 days x S hours/day 1736 hours Breaks: .5 hour/day 109 hours = .5 hours/day x 217 work days 1629 hours per person-year (approximately 203 man-days) The person-years to staff a normal 8-hour week day and 7-day/24-hour staffing can be estimated as presented in Table A.4.3-2. The last column, labeled person-years is Me number of staff persons per year required to staff for Me hours/day and days/week of Me row. Table A.4.3~2 Person-Year Staffing Requirements . .. Hours/day Days/week Weeks/year Hours/year Person years 8 5 52.14 2086 1.28 24 7 52.14 8760 5.38 Assuming no over more relevant historical information is available, Me ITS manual recommends Me following general allocation of maintenance time requirements per intersection for signal systems: Preventive Maintenance70% 36 hours Response Maintenance25% 13 hours Design Modification5% ~ hours Totals 100% 52 hours per ~ntersechon/controller The hours are representative of survey data for average signal system maintenance requirements based on information from a cross section of junsdictions. Thus, Me number of intersections maintainable per technician can be calculated: 1629 52 ~ 31 intersections per technician `NCHRP`Phasc:.rp~` NCHRP3-51 Phase2F~lReport A4-21

OCR for page 453
This number is in general agreement web staffing surveys presented in Reference 8, "Management of Traffic Signal Maintenance," NCH~P Synthesis of Highway Practice ]14, Peter S. Parsonson, (12/84) page 3S, which reported ~ntersecdons per technician ranging from 13 to 81 and an average of 39 intersections per technician. Mean-Time-Be~een-Fa~lure ~F) estimates win supporting data are presented (o 56) for a UTCS system in Washington DC, based on Me following calculations for detectors: ~,, ~_ _J ~J - ~^ ^~49 Component MIFF = # components x hours of test = 497 x 23328 = 104~451 hr. number of failures Subsystem ~BF = hours of test number of failures 11 1 = 23.328 = 210 hr Table A.4.3-3 completes these calculation for other system elements and indicates Mat Me system has a failure every 65.9 hours or every 2-3 days. It should be noted Mat this table provides field verification methodologies for Me statishca]/ma~ematical models presented in Table A.4.2-3. Table A.4.3~3 Component and System MTBF for other system elements Subsystem Number Number Hours Component Subsystem Component Components Failures of Test MTBF tier) MTBF (fur) Central - 1 17 (49~) 8016 472 472 Communication Controllers 111 45 23328 57,452~2, 51 8 Central 51 8016 157 157 Computer (1 49f'l) Detectors 497 1 1 1 23328 104,451 210 Total System 610 354,j 23328 40,210~3) 65.9~4' 1. Adjusted to 23,328 hours 2. Significantly exceeds Apical reports from this dine penod which are typically in Me 5 10,000 hour MIFF range 3. Average over aB components 4. System total differs slightly from NCHRP 114 report due to omission of some components ~:~NC~Phas~pr\ NCHRP3-51 Phase2F~nalReport A4-22 )

OCR for page 453
Mean-Time-to-Repa~ (~) is the time required to return Me failed element to service. FIR is affected by many factors: Number and locationfs) of maintenance shops; Number and location of technicians; Maintenance staff availability: 24-hour, 7-day; 8-hour, 5-day; etc.; Technician travel; Availability of appropriate tools, test equipment, etc.; Time require to troubleshoot and identify cause of failure; Spaling strategy on maintenance vehicle; organizabonai inventories and locations, and suppliers; Time required to obtain replacement parts from spares inventory or suppliers; Time required to replace failed parties) and other required tasks; and Testing/verifying system operation. Thus, M1lR is a function of not only He deployed system and equipment, but also Be maintenance organization strategies and resources. The equipment manufacturer's "on-site" MOOR es~nates must be supplemented with local maintenance organization availability, travel, and log~shc situation. Planning and estimating for maintenance organization smiling can be accomplished when historical data is not readily available by considering fielded equipment count and maintenance staff lime required for preventive maintenance, response maintenance, and design modification. The estimating factors are presented in Table A.4.3-4. J Table A.4~3~ Maintenance Staffing Level Estimating, per Equipment item per Year Preventive Time interval~s): T! (assume days) Hour per year Maintenance Average time required: TR (assume hours) = {365lTI) * TR Response MTBF Hours per year Maintenance MTrR = (~365 days * 24 (these are statistical parameters often hr/dayJJMTBF)*MTTR stated as means with variances) Design Organizational estimates Staff estimates Maintenance u\NC~Phasc2.rps\ NCHRP3-51 Phase2FmalReport A4-23

OCR for page 453
Estimated yearly maintenance staff hours and yearly hours for preventive and response maintenance can then be calculated as: Preventive Response Yearly Hours = N; * (365/Tii )*1~; + ~ Ni * (365*24/M113Fj )*MT~ it, it where Nj = number of ith element in Me system. Staffing requirements can be calculated as: Number maintenance staff = Yearly hours 1629 hr/person As an example, Me manpower estimate to maintain controllers only, in a representative signal system, is presented in Table A.4.3-5. \ Table A.4.3~5 Maintenance Staffing Estimate for Controllers Number of Controllers | 1 1 1 Preventive Maintenance Time Interval (TI) SO ~ rewrnmen~ , Preventive Maintenance Time Required (TR) 3 ~ Resoonse Maintenance MTBF 57 542 coup mean . , , Response Maintenance MTTR 4 hours, mean ~. Yearly Preventive Maintenance hours 1 351 hours N * (365/ Tl )*TR Yearly Response Maintenance Hours 68 hours N * (365*24/MTBF) * MTTR Yearly Total Maintenance Hours 1419 hours Maintenance Staffing 1 person (1419 hours/ 1629 hours/person-year) The staffing estimates for a system would be Me sum of aU hours for ad system elements to be maintained, adjusted as appropriate for other factors such as geographic dispersion, organizational reqliirements, personnel skill requirements, etc. ~:`NC~Phase:.rpr\ NClIRP3-51 Phase2F:nalReport A4-24