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Midwest Research Institute Solutions through science and technology NCHRP Project 15-26 Passing Sight Distance Criteria April 5, 2005.

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Presentation on theme: "Midwest Research Institute Solutions through science and technology NCHRP Project 15-26 Passing Sight Distance Criteria April 5, 2005."— Presentation transcript:

1 Midwest Research Institute Solutions through science and technology NCHRP Project 15-26 Passing Sight Distance Criteria April 5, 2005

2 2 Project Objectives Evaluate the design and operational criteria for determining minimum PSD Modify the existing criteria or develop new criteria

3 3 Project Scope PSD Criteria for Geometric Design AASHTO Green Book PSD Criteria for Marking of Passing and No-Passing Zones MUTCD

4 4 Project Tasks Task 1 – Review current PSD criteria and models COMPLETE Task 2 – Review literature and research in progress COMPLETE Task 3 – Identify factors that potentially contribute to PSD requirements COMPLETE

5 5 Project Tasks Task 4 – Critique PSD criteria and develop work plan COMPLETE Task 5 – Prepare interim report COMPLETE AFTER PANEL DISCUSSIONS

6 6 Project Tasks Task 6 – Execute approved work plan WILL BEGIN WHEN AUTHORIZED Task 7 – Prepare new or modified PSD criteria WILL FOLLOW TASK 6

7 7 Project Tasks Task 8 – Prepare final report WILL FOLLOW TASKS 6 & 7 Task 9 – Prepare and deliver final presentations WLL FOLLOW TASK 8

8 8 Project Schedule Planned Start Date for Phase II: 2/1/05 Actual Start Date for Phase II: 4/6/05 ? Approximately two months behind schedule

9 9 Project Schedule Submit Draft Final Report 2/28/06 Submit Revised Final Report 5/31/06

10 10 Outline of Presentation Review of Current PSD Criteria –Section 2 of interim report Assessment of Current PSD Criteria and Alternative Models –Sections 3 and 4 of the interim report

11 11 Outline of Presentation Potential Work Plans for Execution in Phase II –Section 5 of interim report Priorities and Budget Allocations for Phase II –Section 6 of the interim report

12 12 PSD Design Criteria AASHTO Green Book PSD = d 1 + d 2 + d 3 + d 4 d 1 = P-R time plus initial acceleration d 2 = distance traveled in left lane d 3 = clearance distance d 4 = distance traveled by opposing vehicle = 2/3 d 2

13 13 PSD Design Criteria ASSUMPTIONS Passed vehicle travels at uniform speed Passing vehicle reduces speed and trails the passed vehicle as it enters the passing section (delayed pass)

14 14 PSD Design Criteria Passing driver requires a short period to perceive the passing section and begin to accelerate: –d 1 maneuver time = 3.6 to 4.5 sec (older field data) –d 1 acceleration rate = 1.38 to 1.51 mph/sec (2 to 2.2 ft/sec 2 )

15 15 PSD Design Criteria Passing is accomplished under: –delayed start –hurried return in the face of opposing traffic

16 16 PSD Design Criteria Left-lane distance (d 2 ) –average speed of passing vehicle during left-lane occupancy exceeds speed of passed vehicle by 10 mph at low speeds, passing vehicle above design speed, passed vehicle below design speed at speeds of 55 mph or more, both vehicles below design speed –left-lane occupancy time = 9.3 to 7.3 sec, depending on speed (older field data)

17 17 PSD Design Criteria Clearance distance (d 3 ) –100 to 300 ft, depending upon speed Opposing vehicle distance (d 4 ) –d 4 = d 2 would allow for completion of passing maneuver without need to abort –d 4 = 2/3 d 2 will require abort in some cases

18 18 PSD Design Criteria CRITIQUE AASHTO model is extremely conservative: –If model were d 1 + d 2 + d 3 + d 2, passing driver would know that no abort is required even before beginning to pass –If model d 1 + d 2 + d 3 + 2/3 d 2, an abort is potentially required only very early in the maneuver

19 19 PSD Design Criteria Inclusion of P-R and initial acceleration distance (d 1 ) is very conservative: –passing maneuver could be aborted easily and safely if opposing vehicle appears during d 1 –an opposing vehicle that appears during d 1 is a long way from the passing vehicle

20 20 PSD Design Criteria Ending section of adequate PSD when PSD falls below d 1 + d 2 + d 3 + 2/3 d 2 is extremely conservative: –at the end of the section of adequate PSD, a passing driver has almost enough sight distance to start a pass and complete it without the need to abort –passes can be started safety beyond this point, if the passing driver can abort the maneuver, if an opposing vehicle appears

21 21 PSD Design Criteria Assumption of passed vehicle speed substantially less than design speed, particularly for design speeds of 55 mph or more, is not conservative Assumption of constant speed differential between passing and passed vehicles, independent of design speed is questionable

22 22 PSD Marking Criteria MUTCD presents warrants for no-passing zones passing zones merely happen where no- passing zones are not warranted where the distance between successive no- passing zones is less than 400 ft, no-passing markings should connect the zones

23 23 PSD Marking Criteria There is no quantitative model for the MUTCD criteria MUTCD criteria were first developed in a 1940 AASHO policy

24 24 PSD Marking Criteria MUTCD/1940 AASHO ASSUMPTIONS no P-R time needed for pass initiation because passing driver can abort the maneuver if an opposing vehicle appears speed of passing vehicle is equal to 85 th percentile speed or posted or statutory speed limit

25 25 PSD Marking Criteria speed differential between passing and passed vehicle ranges from 10 to 25 mph, with higher speed differentials at higher speeds speed of opposing is 5 to 15 mph less than speed of passing vehicle, with higher speed differentials at higher speeds compromise between PSD values for delayed and flying passes

26 26 PSD Marking Criteria CRITIQUE Speed of passing vehicle equal to 85 th percentile speed or speed limit is conservative: –many passing vehicles may travel faster

27 27 PSD Marking Criteria Speed differential between passing and passed vehicles that increases with increasing speed is not conservative –it seems more likely that speed differential would decrease as the speed of the passed vehicle increases

28 28 PSD Marking Criteria Speed of opposing vehicle less than 85 th percentile speed or speed limit seems unrealistic Consideration of flying passes is not conservative Minimum 400-ft passing zones are not compatible with delayed passes on high-speed highways

29 29 Comparison of PSD Criteria

30 30 Comparison of PSD Criteria Speed (mph) AASHTO PSD (ft) MUTCD PSD (ft) 401,470 600 501,835 800 602,1351,000 702,4801,200

31 31 Comparison of PSD Criteria AASHTO Criteria Driver eye height =3.50 ft Target height =3.50 ft (reduced from 4.25 ft in 2001) MUTCD Criteria Driver eye height = 3.50 ft Target height = 3.50 ft

32 32 International PSD Criteria PSD Design Criteria @ 100 km/h Australia (more than US at beginning of PSD, less than US at end of PSD) Austria (about the same as the US) Britain (less than US at beginning of PSD, much less at end of PSD) Canada (about the same as the US) Germany (slightly less than the US) Greece (slightly less than the US) South Africa (about the same as the US)

33 33 International PSD Criteria PSD Marking Criteria @ 100 km/h Australia (slightly less than the US) Britain (less than the US) Canada (more than the US) South Africa (about the same as the US)

34 34 Safety Performance for Passing Maneuvers HSIS study (1994) found that: –passing-related accidents constitute 2% of total non-intersection accidents on rural two-lane highways –passing-related accidents are more severe than non-passing related accidents

35 35 Safety Performance for Passing Maneuvers Fatal and serious injury accidents: –13.9% of passing-related accidents – 9.4% of total accidents Passing-related accidents are estimated to constitute 3% of total fatal and serious injury accidents on rural two-lane highways

36 36 Safety Performance for Passing Maneuvers FARS 2003 data: –13,000 fatal accidents/year at non- intersection locations on rural two-lane highways –if 3% are passing-related accidents, there are 390 fatal passing-related accident per year Not all passing-related accidents involve limited PSD

37 37 Safety Performance for Passing Maneuvers COLLISION TYPES FOR PASSING-RELATED ACCIDENTS Single-vehicle ROR 30% Sideswipe, same direction 25% Sideswipe, opp direction 7% Rear-end 17% Head-on 7% Other/unknown 15%

38 38 Safety Performance for Passing Maneuvers Safety record of passing-related accidents is generally good It is unlikely that safety performance of rural two-lane highways can be modified significantly by changing PSD criteria Potential cost-effectiveness of changes to PSD criteria need be investigated

39 39 PSD Criteria in Relation to the Good Safety Record of Passing Maneuvers PSD design criteria are conservative PSD values used in marking criteria are more appropriate than they seem Short 400-ft passing zones may not be often used for high-speed passes Most drivers may be conservative in making passing judgments A buffer area is present downstream of every passing zone

40 40 d 1 + 2d 2 + d 3 Critical Position Position of the Passing Vehicle Passing Sight Distance Sight distance needed to abort pass Sight distance needed to complete pass d 1 + d 2 0 Conceptual Presentation of the Changes in Sight Distance Needed to Complete or Abort a Passing Maneuver as the Passing Maneuver Progresses d 1 + d 2 + d 3 + 2 / 3 d 2 AASHTO MUTCD

41 41 Critical Position in the Passing Maneuver Models based on the critical position assume that drivers may abort the passing maneuver until the critical position is reached

42 42 Critical Position in the Passing Maneuver Critical position concept –any passing driver who has not yet reached the critical position must have sufficient PSD to abort the maneuver –any passing driver who has passed the critical position must have sufficient PSD to complete the maneuver –any passing driver at the critical position must have sufficient PSD to complete or abort the maneuver

43 43 Critical Position in the Passing Maneuver Two models based on the critical position provide PSD values similar to the MUTCD: –Glennon (1988) –Hassan et al. (1996)

44 44 Critical Position in the Passing Maneuver First recognized by VanValkenburg and Michael (1971) –they called it the point of no return –they visually identified the critical position as occurring when the vehicles are approximately abreast (rear bumper of passed vehicle opposite middle of passing vehicle) –field measurements of distance traveled by the passing vehicle from the abreast position to the completion of the pass

45 45 Critical Position in the Passing Maneuver Weaver and Glennon (1972) –defined the critical position as the point at which the time required to complete the maneuver is equal to the time required to abort the maneuver –proposed a PSD model that did not incorporate this definition –stated that critical position occurs when vehicles are approximately abreast

46 46 Critical Position in the Passing Maneuver Harwood and Glennon (1976) –defined the critical position as the point at which the sight distance required to complete the maneuver is equal to the sight distance required to abort the maneuver –proposed a PSD model that did not incorporate this definition

47 47 Critical Position in the Passing Maneuver Lieberman (1982) –used critical position concept in modeling PSD –defined needed PSD as d 7 + PSD c –d 7 is distance to from the start of the pass to the critical position –formulated a model to determine the relative position of the passing and passed vehicles at the critical position ( Δ c ) –model for Δ c appears incomplete

48 48 Critical Position in the Passing Maneuver Saito (1984) –postulated that there are two possible locations of the critical position in a passing maneuver: head-to-tail position (head of passing vehicle opposite tail of passed vehicle) abreast position (passing vehicle alongside passed vehicle) –model inputs are not fully stated

49 49 Critical Position in the Passing Maneuver Glennon (1988) –formulated PSD model based on equivalent sight distance to complete and abort a passing maneuver from the critical position –included an explicit model to calculate the relative positions of the passing and passed vehicles at the critical position ( Δ c )

50 50 Critical Position in the Passing Maneuver Glennon (1988) –included terms for the lengths of the passing and passed vehicles –incorporated speed differential between passing and passed vehicles that decreases as speed increases –deceleration rate in abort maneuver = 8 ft/sec 2 (as opposed to 11 ft/sec 2 in SSD)

51 51 Critical Position in the Passing Maneuver

52 52 Critical Position in the Passing Maneuver Harwood and Glennon (1989) –used Glennon model with some changes in input data from Glennon (1988): passenger car length, 19 ft rather than 16 ft truck length 75 ft smaller speed differential for truck as the passing vehicle lower deceleration rate in aborting a pass, 5 ft/sec 2 for a truck, as opposed to 8 ft/sec 2 for a passenger car

53 53 Critical Position in the Passing Maneuver

54 54 Critical Position in the Passing Maneuver Rilett at al. (1989) –recommended a minimum speed for passing abort maneuver: V d – 2m –inclusion of a minimum speed substantially lengthens PSD values –Good et al. (1991) stated that it is unreasonable to expect that, in the face of an opposing vehicle, the passing driver would decelerated to V d – 2m and then continue at constant speed

55 55 Critical Position in the Passing Maneuver Rilett at al. (1989) –recommended headway after pass abort greater than the 1 sec used by Glennon –1 sec headway appears appropriate for passenger cars –headway greater than 1 sec appears needed for trucks

56 56 Critical Position in the Passing Maneuver Hassan et al. (1996) –recommended two modifications to the Glennon model –recommended incorporation of P-R time for pass abort decision (but this may already be part of the Glennon model) –recommended providing PSD for pass completion where critical position occurs with passing vehicle ahead of passed vehicle (positive value of Δ c )

57 57 Critical Position in the Passing Maneuver

58 58 Critical Position in the Passing Maneuver

59 59 Critical Position in the Passing Maneuver

60 60 Critical Position in the Passing Maneuver

61 61 Buffer Area Downstream of Passing Zones Beginning of passing zone End of passing zone Passing maneuvers in Dir 1 must legally end here Passing maneuvers in Dir 1 can safely end here d 5 Buffer Zone Dir 1 Dir 2

62 62 Buffer Area Downstream of Passing Zones Buffer area results from short zone marking concept: –passes must legally be completed prior to end of zone –drivers can complete passes safely even if in critical position at end of zone –used in 46 of the 50 states

63 63 PSD Marking Criteria HSIS study suggests no major safety problems associated with PSD marking criteria Glennon and Hassan et al. models suggest that MUTCD criteria may be about right, but for the wrong reason Differences between Glennon and Hassan et al. models need to be resolved

64 64 PSD Marking Criteria Cost of changing PSD criteria would be substantial – remeasuring PSD for all two-lane roads with centerlines Cost-effectiveness of potential changes in marking criteria needs to be resolved

65 65 Potential Approach to PSD Marking Criteria IF SUPPORTED BY PHASE II RESULTS Retain MUTCD criteria Offer a better engineering rationale for the MUTCD criteria based on the Glennon model, the Hassan et al. model, or some variation of these models

66 66 Potential Approach to PSD Marking Criteria Consider need to change 400-ft minimum passing zone length

67 67 Short Passing Zones Cant accommodate delayed passes on high- speed highways May accommodate some flying passes May accommodate passing very slow- moving vehicles – tractor on the road May not contribute much to LOS Do drivers use short zones legally? safely?

68 68 Short Passing Zones Jones (1970) –Texas roadways with 70-mph speed limits –field studies at three passing zones with lengths of 400, 640, and 880 ft –comparative data for passing zones with lengths of 1,640 and 2,600 ft

69 69 Short Passing Zones Jones (1970) –defined passing opportunity as: trailing vehicle within 4 car lengths (80 ft) appeared to be awaiting a change to pass average of 125 passing opportunities observed per zone

70 70 Short Passing Zones Length of Passing Zone (ft) Passing Opportunities Accepted Slightly Forced Return Very Severe Return 400 9%37%26% 640 9%45% 0% 880 9%10%0% 1,64023% 2,60040%

71 71 PSD Design Criteria What is the rationale for use of longer PSD values in design than in marking? Does the good safety record of passing maneuvers indicate that current marking criteria are sufficient for safety? Does the use of longer PSD values in design than in marking enhance safety? Should the design process explicitly consider the passing and no-passing zones that will be marked on the completed highway?

72 72 Potential Alternative Approaches to PSD Marking Criteria #1 – Retain current AASHTO Green Book criteria #2 – Use the same PSD criteria for design as for marking #3 – Use PSD criteria for design with a defined relationship to PSD criteria for marking: PSD = X + PSD c

73 73 Potential Alternative Approaches to PSD Marking Criteria #4 - Use the same PSD criteria for design as for marking, but count only passing sections with specified minimum length #5 – Use longer value of PSD to define beginning of passing section and shorter PSD to define end of passing section

74 74 Other Issues Trucks Older drivers

75 75 PSD for Trucks

76 76 PSD for Trucks Passing Scenario PSD needed for design speed of 60 mph (ft) PC passing PC1,025 PC passing Truck1,250 Truck passing PC1,375 Truck passing Truck1,575

77 77 PSD for Trucks Glennon model may need longer headway after abort for truck as passing vehicle Truck can pass PC on any vertical curve where PC can pass a truck – reevaluate in light of changed object heights

78 78 PSD for Trucks PC passing truck or truck passing PC may not be a logical design or marking scenario: –would eliminate some current passing zones and shorten others –would prohibit some passes of PCs that are safe and are currently legal –would reduce LOS –might encourage illegal passes –no indication of safety benefits

79 79 Older Drivers Reduced P-R time Reduced visual acuity Reduced ability to judge distances and speeds Less likely to travel at high speeds Less likely to pass Less aggressive More likely to drive passed vehicle than passing vehicle

80 80 PSD for Older Drivers FHWA Highway Design Handbook for Older Drivers recommended using Green Book PSDs instead of MUTCD PSDs NCHRP Project 20-7(118) recommended caution in implementing this Handbook recommendation

81 81 PSD for Older Drivers Use of longer PSD in marking would eliminate some current passing zones and shorten others Older drivers would still probably be more reluctant than younger drivers to pass Handbook recommendation did not consider the safety cushion provided by the buffer area

82 82 Key Considerations in Changing PSD Criteria Safety considerations (effect on accident frequency and severity) Traffic operational considerations (effect on level of service) Economic considerations (benefits to compensate for increased costs) on both existing and new highways Rationality and consistency of PSD criteria (understanding and acceptance by engineers)

83 83 Potential Phase II Work Plans A – Benefit-cost analysis B – Identify and analyze of passing- related accidents C – Review accident data for field sites D – Safety of passing maneuvers completed beyond the end of a passing zone

84 84 Potential Phase II Work Plans E – Safety and operations of short passing zones F – Field data to quantify parameters of revised PSD models G – Application of PSD criteria to actual terrain H – Traffic operational effects of alternative PSD criteria

85 85 Potential Phase II Work Plans I – Comparison of PSD and SSD criteria J – Guidance on determination of percentage of roadway length with PSD

86 86 A – Benefit-Cost Analysis for Changing PSD Marking Criteria Objective: –determine whether safety benefits could possibly justify cost of remeasuring PSD –need to quantify: cost per mile of remeasuring PSD number of miles of two-lane roads with marked centerlines number of accidents equivalent to cost of remeasuring number of accidents available for reduction

87 87 BIdentification and Analysis of Passing-Related Accidents Objective: Conduct further evaluation of passing-related accidents Identify passing-related accidents in manner similar to HSIS study Obtain more complete severity data Estimate available PSD at accident sites Determine portion of passing zone (or no- passing zone) where accidents occur

88 88 BIdentification and Analysis of Passing-Related Accidents Investigate involvement of trucks Investigate involvement of older drivers in passing, passed, and opposing vehicles Determine weather and pavement conditions under which passing-related accidents occur

89 89 CReview of Accident Data for Field Sites Objective: –Assure that field sites used in Work Plans D and F do not have adverse accident experience –Determine accident experience for field sites in Work Plan E

90 90 DSafety of Maneuvers Completed Beyond the End of a Passing Zone Objective: Determine the extent and consequences of pass completions beyond the end of a passing zone Sites to be used: –passing zone length: 1,000 to 2,500 ft –frequent passing activity –no-passing zone for at least 2,000 ft downstream of passing zone

91 91 DSafety of Maneuvers Completed Beyond the End of a Passing Zone Data to be collected: –percent of maneuvers completed in marked passing and no-passing zones –relative positions of passing and passed vehicles at the end of the passing zone –location of passing vehicles return to normal lane –sight distance at return to normal lane –traffic conflicts or severity of return maneuver Video recording and manual observation

92 92 ESafety and Operations of Short Passing Zones Objectives: Determine whether short passing zones: –create safety problems –contribute substantially to LOS Sites to be used: –passing zone length: 400 to 800 ft Data to be collected: – similar to Work Plan D

93 93 FQuantify Parameters of Revised PSD Models Objective: Quantify parameters of alternative models –speed differential between passing and passed vehicles –distance traveled by the passing vehicle from the beginning of passing zone to the critical position –deceleration rate use in aborting a passing maneuver

94 94 FQuantify Parameters of Revised PSD Models Data collection approach: –Video recording –Manual observation –Traffic classifiers or laser guns for speed measurement

95 95 GApplication of Revised PSD Criteria to Actual Terrain Objective: Investigate effects of changing PSD design and marking criteria Obtain plan and profile data for actual terrain: –as-built plans –Washington HSIS data

96 96 GApplication of Revised PSD Criteria to Actual Terrain How will revised PSD criteria change: –percentage of roadway length that meets design PSD criteria –percentage of roadway length in marked passing zones –lengths of passing zones –PSD available within passing zones Relative contributions of PSD criteria and terrain

97 97 GApplication of Revised PSD Criteria to Actual Terrain Interviews with experienced designers: –choice of target percentage of roadway length that meets design PSD criteria –application of design criteria

98 98 HTraffic Operational Effects of Alternative PSD Criteria Objective: How will revised PSD design and marking criteria affect the level of service on two-lane highways? Tool: TWOPAS traffic operational computer simulation model

99 99 I– Comparison of PSD and SSD Criteria Compare PSD values used in design and marking to SSD values used in design Compare resulting vertical curve lengths in light of: –sight distance criteria –assumed driver eye height –assumed object height

100 100 J– Guidance on Percentage of Roadway Length with Design PSD Provide guidance or recommendations on average frequency of passing opportunities that should be provided on two-lane highways –dependent on traffic volumes –dependent on vehicle mix –needs to implement desired LOS for functional class and terrain

101 101 J– Guidance on Percentage of Roadway Length with Design PSD Added passing lanes along the road may reduce the need for PSD to maintain LOS between passing lanes

102 102 Project Budget Phase I $ 64,066 Phase II 235,934 TOTAL$300,000

103 103 Phase I Budget Budgeted expenditures:$64,066 Actual expenditures (est.) 61,000 Unexpended funds (est.) 3,066

104 104 Allocation of Phase II Budget

105 105 Cost Estimates for Task 7 Work Plans A – Benefit-cost analysis of PSD $ 10,000 marking revisions B – Identification and analysis of 100,000 passing-related accidents C – Review of accident data for 5,000 field study sites D – Safety of passing maneuvers 30,000 completed beyond the end of the passing zone

106 106 Cost Estimates for Task 7 Work Plans E – Safety and operations of$ 30,000 short passing zones F – Field data collection to quantify 50,000 PSD models G – Application of revised PSD 10,000 to actual terrain H – Traffic operational effects 15,000 of alternative PSD criteria

107 107 Cost Estimates for Task 7 Work Plans I – Comparison of PSD and SSD$ 5,000 criteria J – Guidance on determining the 10,000 percentage of roadway length with adequate PSD TOTAL $265,000

108 108 Cost Estimates for Task 7 Work Plans Estimated cost for all 10 work plans = $265,000 Available funds for Task 7 = $181,000 Difference = $84,000

109 109 Potential Modifications Eliminate Work Plan B entirely –saves $100,000 –frees up $16,000 for field work Scale back Work Plan B to $16,000 –existing data bases only –no videolog review –no hard-copy accident report review


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