Capacity & level of service (transportation engineering)
- 2. Design Speed 2 AASHTO defines design speed as follows: “Design speed is a selected speed used to determine the various geometric features of the roadway. Or “The maximum safe speed that can be maintained over a specified section of highway when the conditions are so favorable that the design features of the highway govern” Or The service flow rate that is used for design of specific road facility is known as Design Capacity
- 3. Design Speed 3 The assumed design speed should be a logical one with respect to the; • Topography (Terrain) • Anticipated operating speed • The adjacent land use • Functional classification of the highway. • Terrain Types? • Level, Rolling, Mountainous,? • Pedestrian?
- 4. In selection of design speed, every effort should be made to attain a desired combination of safety, mobility, and efficiency within the constraints of , environmental quality Economics • Aesthetics, and social or political impacts. Once the design speed is selected, all of the pertinent highway features should be related to it to obtain a balanced design. Design Speed 4
- 5. Capacity is defined as the maximum number of vehicles, passengers, or the like, per unit time, which can be accommodated under given conditions with a reasonable expectation of occurrence Design Capacity 5 Capacity is independent of the demand. It speaks about the physical amount of vehicles and passengers a road can afford. It does not depend on the total number of vehicles demanding service. On the other hand, it depends on traffic conditions, geometric design of the road etc. For example, a curved road has lesser capacity compared to a straight road. Capacity is expressed in terms of units of some specific thing (car, people, etc.), so it also does depend on the traffic composition.
- 6. 1. Traffic conditions: It refers to the traffic composition in the road such as the mix of cars, trucks, buses etc in the stream. It also include peaking characteristics, proportions of turning movements at intersections and the like. 2. Road way characteristics: This points out to the geometric characteristics of the road. These include lane width, shoulder width, lane configuration, horizontal alignment and vertical alignment. 3. Control conditions: This primarily applies to surface facilities and often refer to the signals at intersections etc. Design Capacity 6
- 7. Factors Affecting Highway Capacity 1. Lane width 2. Width of shoulder 3. Lateral clearance 4. Commercial vehicles 5. Road alignment and geometry (curves, Super elevation etc) 6. Existence of intersections. 7. One way or two way traffic and number of lanes 8. Drivers and vehicular characteristics 9. Single type or mixed traffic 10. Flow speed 11. Weather condition 12. Road Side Parking 13. Presence of pedestrians Design Capacity 7
- 8. Flow Rate/ Hourly Volume 8 rate of flow q = number of vehicles during observation veh/h observation time Flow is one of the most common traffic parameters. Flow is the rate at which vehicles pass a given point on the roadway, and is normally given in terms of vehicles per hour.
- 9. 9 Various Types of Traffic Volumes Daily Volume Average Annual Daily Traffic (AADT) Average Annual Weekday Traffic (AAWT) Average Daily Traffic (ADT) Average Weekday Traffic (AWT) Hourly Volume Subhourly Volume Peak Hour Volumes
- 10. Types of Traffic Volumes 10 Average Annual Daily Traffic (AADT) Average 24-hour traffic volume at a location over a full 365-day year, which is the total number of vehicles passing the location divided by 365. Average Annual Weekday Traffic AAWT) Average 24-hour traffic volume on weekdays over a full year, which is the total weekday traffic volume divided by 260
- 11. Types of Traffic Volumes 11 Average Daily Traffic (ADT) Average 24-hour traffic at a location for any period less than a year (e.g. six months, a season, a month, a week or even two days) Average Weekday Traffic (AWT) Average 24-hour traffic volume on weekdays for any period less than a year
- 12. Peak Hour Volume 12 Peak hour volumes are sometimes estimated from projections of the AADT. Sometimes PHV is referred as “directional design hour volume” (DDHV) DDHV = AADT * K * D Where: K – proportion of daily traffic occurring during the peak hour D – proportion of peak hour traffic traveling in the peak direction of flow
- 16. Capacity & LOS 16 Capacity analysis: tries to give a clear understanding of how much traffic a given transportation facility can accommodate. Level of service:tries to answer how good is the present traffic situation on a given facility. Thus it gives a qualitative measure of traffic, where as capacity analysis gives a quantitative measure of a facility.
- 18. Ideal Conditions For LOS Determination 18 Chapter 21 of the Highway Capacity Manual For rural and suburban multilane highways Assumptions (Ideal Conditions, all other conditions reduce capacity): Only passenger cars No direct access points A divided highway FFS > 60 mph
- 19. Base Conditions For LOS Determination 19 Base Conditions For Freeways use BFFS = 70 mi/h (110 km/h) for freeways in urban areas use BFFS = 75 mi/h (120 km/h) for freeways in Rural areas 6 ft. (1.8 m) minimum right shoulder clearance 2 ft. ( 0.6 m) minimum median lateral clearance 12 ft. (3.6m ) minimum lane width Only passengers cars in traffic stream Five or more lane in each traffic stream 2 mi (3.2 km) or greater interchange spacing Drivers should be road familiar
- 20. Base Conditions For LOS Determination 20 Base Conditions For Multilane Highways OR Highways use BFFS = 60 mi/h (100 km/h) for highways in urban areas use BFFS = 65 mi/h (105 km/h) for highways in Rural areas Use BFFS = Posted speed in mi/h + 5 mi/h 12 ft. (3.6 m) minimum lane width 12 ft. (3.6 m) minimum total lateral clearance from outside objects (right shoulder and median) in the travel direction Only passenger cars in the traffic stream No direct access points in along the roadway Divided highway Drivers should be familiar roadway users
- 21. Example (LOS Determination) 21 Determine the level of service (LOS) for a divided 4-lane highway for the following parameters. BFFS= 60 mph, LW = 10 ft, TLC = 10 ft 20access points/ mile, for 10% heavy trucks on rolling terrain, base volume is 2,500 veh/hour in one direction , PHF = 0.9, Non- familiar users fp = 0.85
- 22. Example (LOS Determination) 22 Source: HCM, 2000
- 23. Example (LOS Determination) 23 Source: HCM, 2000 Lane Width: • Base Conditions: 12 foot lanes How much does use of 10-foot lanes decrease free flow speed? Flw = 6.6 mph
- 24. Example (LOS Determination) 24 Lateral Clearance Distance to fixed objects Assumes >= 6 feet from right edge of travel lanes to obstruction >= 6 feet from left edge of travel lane to object in median TLC = LCR + LCL TLC = total lateral clearance in feet LCR = lateral clearance from right edge of travel lane LCL= lateral clearance from left edge of travel lane Source: HCM, 2000
- 25. Example (LOS Determination) 25 Flc = 0.4 mph Source: HCM, 2000
- 26. Example (LOS Determination) 26 Source: HCM, 2000 fm: Accounts for friction between opposing directions of traffic in adjacent lanes for undivided No adjustment for divided, fm = 0
- 27. Example (LOS Determination) 27 Source: HCM, 2000 Fa accounts for interruption due to access points along the facility Example: if there are 20 access points per mile, what is the reduction in free flow speed? fa = 5.0 mph
- 28. Example (LOS Determination) 28 Source: HCM, 2000 BFFS = free flow under ideal conditions FFS = free flow adjusted for actual conditions From previous examples: FFS = 60 mph – 6.6 mph - 0.4 mph – 0 – 5.0 mph = 48 mph ( reduction of 12 mph)
- 29. Example (LOS Determination) 29 Calculate Flow Rate Heavy vehicles affect traffic & Slower stream larger fhv increases number of passenger vehicles to account for presence of heavy trucks
- 30. Example (LOS Determination) 30 f(hv) General Grade Definitions: Level: combination of alignment (horizontal and vertical) that allows heavy vehicles to maintain same speed as pass. cars (includes short grades 2% or less) Rolling: combination that causes heavy vehicles to reduce speed substantially below P.C. (but not crawl speed for any length) Mountainous: Heavy vehicles at crawl speed for significant length or frequent intervals
- 31. Example (LOS Determination) 31 f(hv) General Grade Definitions: Level: combination of alignment (horizontal and vertical) that allows heavy vehicles to maintain same speed as pass. cars (includes short grades 2% or less) Rolling: combination that causes heavy vehicles to reduce speed substantially below P.C. (but not crawl speed for any length) Mountainous: Heavy vehicles at crawl speed for significant length or frequent intervals
- 32. Example (LOS Determination) 32 Example: for 10% heavy trucks on rolling terrain, what is Fhv? For rolling terrain, ET = 2.5 Fhv = _________1_______ = 0.87 1 + 0.1 (2.5 – 1)
- 33. Example (LOS Determination) 33 Driver Population Factor (fp) Non-familiar users affect capacity fp = 1, familiar users 1 > fp >=0.85, unfamiliar users Calculate vp Example: base volume is 2,500 veh/hour PHF = 0.9, N = 2 fhv from previous, fhv = 0.87 Non-familiar users, fp = 0.85 vp = _____2,500 vph _____ = 1878 pc-ph-pl 0.9 x 2 x 0.87 x 0.85
- 34. Example (LOS Determination) 34 Calculate Density Example: for previous D = _____1878 vph____ = 39.1 pc/mi/lane 48 mph
- 35. Example (LOS Determination) 35Also, D = 39.1 pc/mi/ln, LOS E
- 36. Example (LOS Improvement) 36 What can we change in a design to provide an acceptable LOS? • Lateral clearance (only 0.4 mph) • Lane width • Number of lanes Design Decision
- 37. Example (LOS Improvement) 37 Source: HCM, 2000 Lane Width (Example) How much does use of 10 foot lanes decrease free flow speed? Flw = 6.6 mph
- 38. Example (LOS Improvement) 38 Recalculate Density Example: for previous (but with 12 ft lanes and TLC = 12 ft) D = _____1878 vph____ = 34.1 pc/mi/lane 55 mph
- 39. Example (LOS Improvement) 39 39 LOS = E Now D = 34.1 pc/mi/ln, on border of LOS E
- 40. Example (LOS Improvement) 40 Recalculate vp, while adding a lane Example: base volume is 2,500 veh/hour PHF = 0.9, N = 3 fhv from previous, fhv = 0.87 Non-familiar users, fp = 0.85 vp = _____2,500 vph _____ = 1252 pc/ph/pl 0.9 x 3 x 0.87 x 0.85
- 41. Example (LOS Improvement) 41 Recalculate vp, while adding a lane Example: base volume is 2,500 veh/hour PHF = 0.9, N = 3 fhv from previous, fhv = 0.87 Non-familiar users, fp = 0.85 vp = _____2,500 vph _____ = 1252 pc/ph/pl 0.9 x 3 x 0.87 x 0.85
- 42. Example (LOS Improvement) 42 Calculate Density Example: for previous D = _____1252 vph____ = 26.1 pc/mi/lane 48 mph
- 43. Example (LOS Improvement) 43 LOS = D Now D = 26.1 pc/mi/ln, LOS D (almost C)
- 44. Example 02 (LOS Determination) 44
- 45. Example 02 (LOS Determination) 45