ECGD 4121 – Transportation Engineering I Lecture 6

1. Faculty of Applied Engineering and Urban Planning Civil Engineering Department 2nd Semester 2008/2009 ECGD 4121 – Transportation Engineering I Lecture 6

2. Capacity & Level of Service

3. Demand • Amount of traffic desiring to use a facility • When no queuing is involved • Demand = traffic volume

4. Capacity Maximum hourly rate persons or vehicles can be reasonable expected to traverse a point or uniform section during a given time • Under prevailing roadway conditions • Different for different facilities (freeway, multilane, 2-lane rural, signals) • Assuming no downstream constraints • Backing up of traffic

5. Delay • Stopped time delay: time vehicle waits in queue (only time when vehicle is fully stopped) • Approach delay (signalized intersections): includes stopped time delay and includes time lost for deceleration and to accelerate back to desired speed • Time-in-queue delay (intersection): total time from when a vehicle joins a queue until it discharges from the stop bar (includes time creeping up • Travel time delay: difference between drivers expected travel time and the actual time taken • Control delay (intersection): delay caused by a control device, ~ equal to time-in-queue plus acceleration/deceleration

6. Flow Characteristics • Undersaturated • Traffic flow that is unaffected by upstream or downstream conditions • Queue discharge • Traffic flow that has just passed through a bottleneck and is accelerating back to the FFS of the freeway • Oversaturated • Traffic flow that is influenced by the effects of a downstream bottleneck

7. V/C • Volume to capacity • Ratio of current or projected demand flow to capacity of facility • Measure of sufficiency of capacity

8. V/C • V/C = 1 indicates all supply (capacity is used) • In theory V/C cannot be > 1.0 • In reality V/C > 1.0 can occur • Capacity is an estimate • Users will more efficiently use system when V/C is high

9. Free Flow Speed (FFS) The mean speed of passenger cars that can be accommodated under low to moderate flow rates on a uniform freeway segment under prevailing roadway and traffic conditions.

10. Free Flow Speed (FFS) Factors affecting free-flow speed • Lane width • Lateral clearance • Number of lanes • Interchange density • Geometric design

11. Level of Service (LOS) • Concept: a qualitative measure for describing operational conditions within a traffic stream and their perception by drivers and/or passengers • LOS’s represent range of operating conditions defined by measures of effectiveness (MOE)

12. Level of Service (LOS) • Describes operational conditions of facility • Speed • Travel time • Freedom to maneuver • Traffic interruptions • Comfort • convenience • A: best • F: worst

13. Service Flow Rate • Maximum flow rate at each level of service from level A to level E • Difficult to predict for level F • Maximum hourly rate at which vehicles can traverse a roadway under prevailing conditions • Usually based on 15 min periods

14. Base Conditions • Standard conditions • Non-standard conditions are adjusted to match • Conditions • Good weather only passenger cars • Users familiar with the system • No impediments to flow • 12 foot lanes • flat terrain • Dry pavement

15. Level of Service Measures • Basic freeway segments: density • Signalized intersections: delay • Pedestrians: space per ped, flow rate, speed • Rural two lane facilities: percent time spent following • Urban streets (not intersections): average travel speed

16. LOS - A (Freeway) • Free flow conditions • Vehicles are unimpeded in their ability to maneuver within the traffic stream • Incidents and breakdowns are easily absorbed

17. LOS - B • Flow reasonably free • Ability to maneuver is slightly restricted • General level of physical and psychological comfort provided to drivers is high • Effects of incidents and breakdowns are easily absorbed

18. LOS - C • Flow at or near FFS • Freedom to maneuver is noticeably restricted • Lane changes more difficult • Minor incidents will be absorbed, but will cause deterioration in service • Queues may form behind significant blockage

19. LOS - D • Speeds begin to decline with increasing flow • Freedom to maneuver is noticeably limited • Drivers experience physical and psychological discomfort • Even minor incidents cause queuing, traffic stream cannot absorb disruptions

20. LOS - E • Capacity operations are volatile, virtually no usable gaps • Vehicles are closely spaced • Disruptions such as lane changes can cause a disruption wave that propagates throughout the upstream traffic flow • Cannot dissipate even minor disruptions, incidents will cause breakdown

21. LOS - F • Breakdown or forced flow • Occurs when: • Traffic incidents cause a temporary reduction in capacity • At points of recurring congestion, such as merge or weaving segments • In forecast situations, projected flow (demand) exceeds estimated capacity

22. Design Level of Service This is the desired quality of traffic conditions from a driver’s perspective (used to determine number of lanes) • Design LOS is higher for higher functional classes • Design LOS is higher for rural areas • LOS is higher for level/rolling than mountainous terrain • Other factors include: adjacent land use type and development intensity, environmental factors, and aesthetic and historic values • Design all elements to same LOS (use HCM to analyze)

23. Design Level of Service (LOS)

24. Freeways: Capacity (Free-Flow Speed) 2,400 pcphpl (70 mph) 2,350 pcphpl (65 mph) 2,300 pcphpl (60 mph) 2,250 pcphpl (55 mph) Ideal Capacity

25. Multilane Suburban/Rural 2,200 pcphpl (60 mph) 2,100 (55 mph) 2,000 (50 mph) 1,900 (45 mph) 2-lane rural – 2,800 pcph Ideal Capacity

26. Acceptable Degree of Congestion • Demand  capacity, even for short time • 75-85% of capacity at signals • Dissipate from queue at 1500-1800 vph • Afford some choice of speed, related to trip length • Freedom from tension, esp long trips, < 42 veh/mi. • Practical limits - users expect lower LOS in expensive situations (urban, mountainous)

27. Multilane Highways • Chapter 21 of the HCM, 2000 Edition • 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 • Represents highest level of multilane rural and suburban highways

28. Multilane Highways Intended for analysis of uninterrupted-flow highway segments • Signal spacing > 2.0 miles • No on-street parking • No significant bus stops • No significant pedestrian activities