HIGHWAY CAPACITY STUDIES. Lecture Notes in Civil Engineering

Review LOS definition and determinants

Define capacity and relate to “ideal”
Capacities

Review calculating capacity using HCM
procedures for basic freeway section

Focus on relations between capacity, level of-
service, and design.

1. CVE 719 MATERIALS AND METHODS FOR THE CONSTRUCTION OF HIGHWAYS ENGR. DR. ANDERSON ETIKA DEPARTMENT OF CIVIL ENGINEERING CRUTECH, CALABAR LECTURE 1

2. CVE 719: MATERIALS AND METHODS FOR THE CONSTRUCTION OF HIGHWAYS COURSE CONTENT* Highway CAPACITY STUDIES INTERSECTIONS AND INTERCHANGE INTERSECTION DESIGN Delay studies ACCIDENT STUDIES TRAFFIC CONTROL SYSTEMS * This course is co-taught by Engr. Dr. D. EWA, thus this is only part of the course content to be taught by ENGR. DR. A. ETIKA*

3. HIGHWAY CAPACITY STUDIES

4. OBJECTIVES Review LOS definition and determinants Define capacity and relate to “ideal” Capacities Review calculating capacity using HCM procedures for basic freeway section Focus on relations between capacity, level of- service, and design . HIGHWAY CAPACITY STUDIES

5. INTRODUCTION How much traffic can a given facility accommodate? Under what operating conditions can it accommodate that much traffic?

6. TYPES OF APPROACHES There are two different approaches to determining the capacity of a highway ‘Level of Service Approach Capacities for roads of various sizes and widths

7. LEVEL OF SERVICE A qualitative measure describing operational conditions within a traffic stream and their perception by drivers and/or passengers. It gauges the level of congestion on a highway in terms of variables such as travel times and traffic speed. The Highway Capacity Manual ( TRB , 1994) list 6 levels of services ranging from A (best) to F (Worst). Levels represent range of operating conditions defined by measures of effectiveness (MOE).

8. LOS A (Freeway/MOTORWAY) Free flow conditions (traffic flow is virtually zero) Vehicles are unimpeded in their ability to manoeuver within the traffic stream. Incidents and breakdowns are easily absorbed Comfort and convenience levels are high

9. LOS B Reasonable free-flowing conditions Ability to manoeuver is slightly restricted General level of physical and psychological comfort provided to drivers are still high Effects of incidents and breakdowns are easily absorbed

10. LOS C Delivers stable flow conditions Freedom to maneuver is noticeably Restricted Lane changes more d ifficult Minor incidents will be absorbed, but will cause deterioration in service Queues may form behind significant Blockage Speed chosen by the driver is substantially affected by that of other vehicles. Comfort and convenience has decreased perceptibility at this level.

11. LOS D Speeds begin to decline with increasing flow. Severe restriction to driver’s ability to manoeuvre Drivers experience physical and psychological discomfort Even minor incidents cause queuing, traffic stream cannot absorb disruptions

12. LOS e Represent the level at which the capacity of the highway has been reached . Traffic flow conditions are best described as unstable Operations are volatile, virtually no usable gaps Drivers levels of comfort are very poor Even minor incidents causing extensive queuing and breakdown

13. LOS F This describes the state of breakdown or forced flow with flows exceeding capacity. Operating conditions are highly unstable Constant queuing and traffic moving on a ‘’Stop-Go” basis Drivers comfort level is extensively poor

14. LOS Operating conditions can be expressed graphically with reference to basic speed flow relationship. At LOS A, speed is near its max value, restricted only by road geometry and low flows. At LOS D, flows are maximised with speed at 50% of its Max value. LOS F, denotes the “breakdown condition at which both speeds and flow tend towards zero.

15. DEFINITION OF TERMS To determine a road’s LOS, a comprehension of the relationship between hourly volumes, peak-hour factor and service flow is vital. Hourly-volume ( V ) = The highest volume within a 24-h period. Peak-hour factor (PHF)= The ratio of the hourly volume to the peak 15-min flow ( V 15 ) enlarged to an hourly value PHF = V / V 15 * 4 Service flow (SF) = The peak 15-min flow ( V 15 ) enlarged to an hourly value . SF = V 15 * 4

16. DESIGN OF LEVEL OF SERVICE This is the desired quality of traffic condition 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)

17. DESIGN OF LEVEL OF SERVICE

18. M aximum service flow rates for multilanes The HCM generates max flow values obtainable on multilane highways, given a certain speed limit and prevailing LOS The values assume that ideal conditions exist ( i.e all carriageways have a standard 3.65m, there are no obstructions within 1.83m of the edge, there are no heavy good vehicles, buses or recreational vehs on the road, driver population consists of regular weekday drivers The road is divided by physical barrier Roads are rural based No pedestrians No on-street parking

19. M aximum service flow rates for multilanes In an Ideal conditions, Max service flow, Where N= is the number of lanes in each direction, C j = is the capacity of a standard highway lane for a given design speed j. Table 1

20. Table 2

21. EXAMPLE 1

22. EXAMPLE 1

23. SOLUTION

24. SOLUTION Under the prevailing ideal conditions, therefore, with reference to table 2, the ratio of flow to capacity is greater than 0.54 but less than 0.7. The highway thus provides level of service C.

25. In a non- Ideal conditions, Max service flow, When lane widths are narrower than 3.65 Lighting post or any such obstruction are closer than 1.83 from the edge of the travelled pavement pavement , an adjustment factor f w must be introduced.