Highway Capacity and LOS. Reading Assignment: pgs

Transcription

1 Highway Caacity and LOS Reading Assignment: gs We know from the revious section that traffic flows fairly well when Demand < Caacity. However, when demand aroaches caacity, we begin to exerience lower seeds, congestion. hus in lanning and design of highways we are interested in designing facilities that will oerate below caacity. We need a good estimate of caacity for this. Caacity analysis involves the quantitative evaluation of the caability of a freeway section to carry traffic. We will be concerned in this section with determining the level of service articular freeways rovide, and in designing a freeway to oerate at an accetable LOS. Factors Contributing to LOS: raffic volume is the rincial factor affecting LOS, however, several other factors contribute. 1. Lane width When narrower that 12 ft, traffic flow is restricted. 2. Lateral obstructions When objects are located near the edge of the roadway or in the median, this has the effect of reducing lateral sacing because drivers tend to move away from the object. Effect is eliminated if object is < 6 inches tall and 6 feet from the roadway edge. Lateral clearances are based on safety considerations, not volume. 3. raffic comosition Presence of trucks, buses, rv s, in a traffic stream (anything other than assenger cars) reduces flow due to size, oerating characteristics, and interactions with other vehicles.

2 4. Grade Effect of grade deends on both length and sloe of grade. raffic is significantly affected when grades are 3% and ¼ mile or longer, or grades <3% are ½ mi or longer. 5. Seed Sace mean seed is considered in LOS analysis because flow has a significant effect on seed. Six LOS have been established A-F (A is best). HCM establishes rocedures for determining LOS. We are going to analyze simlified cases of basic freeway segments and only basic tyes of terrain and trucks, even though rocedures and info for analysis of much more comlex situations are outlined in HCM. Basic Freeway Segments: Zone of influence for ram junctions: - On-rams: 500 ft ustream and 2500 ft downstream - Off-rams: 2500 ft ustream, and 500 ft downstream Level of service A Seed of vehicle is controlled only by desires of the driver and the revailing conditions. LOS B raffic still under free-flow conditions, may exerience some restrictions in lane changes. LOS C- Seeds are still near free-flow, however, flow is heavier, and any increase in volume immediately results in decrease of service.

3 LOS D Flow is aroaching an unstable level. Minor incidents may result in queuing. LOS E Flow is unstable (at or near caacity). Lane changes will disturb traffic stream. Minor incidents cause extensive queuing. LOS F Oeration is under forced or breakdown conditions. Ideal conditions for maximum service flow rate: - Minimum interchange sacing 2 miles - Only assenger cars - Lane widths 12 feet - Lateral obstructions 6 ft from roadway edge - Level terrain (grades < 2%) - Drivers tyical of weekday (regular) traffic - 10 or more lanes in urban areas **Criteria removed in HCM 2010 For LOS analysis, we want to convert revailing conditions to equivalent base conditions. For base conditions, a free-flow seed of 70 mh or greater is required. Basic Equation: FFS BFFS f LW f LC f N - f ID Where: FFS estimated free flow seed in mh. BFFS estimated base free flow seed in mh (75 mh for rural freeways, 70 mh for urban based on HCM recommendations). f LW adjustment for lane width (if less than 12 ft), mh.

4 Flow Rate: f LC adjustment for right side lateral clearance ( if less than 6 ft), mh. f N adjustment for # of lanes (if less than 5 in one direction), mh. f ID adjustment for interchange density if < 2 mi, mh. V v PHF N f f Where: v 15-minute assenger-car equivalent flow rate (c/h/ln) V hourly volume in the given direction of flow (vh) PHF eak-hour factor N number of lanes in the given direction of flow f an adjustment factor for th resence of heavy vehicles f an adjustment factor to account for the fact that all drivers of the facility may not be commuters or regular users. *Basis for analysis is eak 15 min flow rate. PHF Peak Hour Volume Peak Hour volume Peak 15 min. flow rate 4 Peak 15 min. Volume Examle: ime (min.) Count (veh) Heavy Vehicle Effects: (trucks, buses, rv s) hese do not erform as well as assenger cars, so they are thought of in assenger car equivalents on a freeway. his equivalent value deends on the tye of terrain: level, rolling, mountainous. f 1+ P 1 + P R R 1+ P 1

5 P, P R roortion of trucks and buses, and RV s E, E B, E R PCEs for trucks and buses, and RV s *We will assume that there are no RV s in our traffic stream, thus P R 0, yielding the above version of the correction factor equation. Analysis is based on general extended freeway segment Level heavy vehicles maintain same seed as c s (grade <2%). Rolling s travel at seeds lower than c. Mountainous s oerate at crawl seed for significant distances. When conditions are very severe, we will instead base on grade and length of grade. Restrictions for use: No grade < 3% for longer than ½ mile. No grade 3% for longer than ½ mile. Determining LOS: Grahical use grah with your calculated v, and your free flow seed. You can also estimate average seed from the grah. able rocedure calculate v, look u LOS for your free-flow seed, your v can not be greater than MSF, or you dro to a lower LOS. MSF i maximum ossible flow rate we can have to still be at level of service i. Determining Caacity: Caacity MSF E x PHF x N x f x f *this is total caacity. Other caacity can be determined to remain in a secific level of service using MSF i. Determining Lane Requirements: Based on design level of service. Iterative rocedure. - Use LOS B for rural freeways - Use LOS C for urban freeways (may be forced to D due to land restrictions). N DD MSF PHF f i f When you calculate design N, you need to round u, otherwise you will dro to a lower LOS.