Lecture 8: LOS Freeway PDF

Capacity and Level of Service (LOS) Analysis 1 Introduction A freeway may be defined as a divided highway with full control of access and two or more lanes for the exclusive use of traffic in each direction. Freeways provide uninterrupted flow. There...

Capacity and Level of Service (LOS) Analysis 1 Introduction A freeway may be defined as a divided highway with full control of access and two or more lanes for the exclusive use of traffic in each direction. Freeways provide uninterrupted flow. There are no signalized or stop-controlled at-grade intersections, and direct access to and from adjacent property is not permitted. Access to and from the freeway is limited to ramp locations. Opposing directions of flow are continuously separated by a raised barrier, an at-grade median, or a raised traffic island. 2 Basic Freeway Segment 3 Performance Measures A basic freeway segment is typically characterized by three main performance measures: 1. density in terms of passenger cars per kilometer per lane, 2. speed in terms of mean passenger car speed, and 3. volume-to-capacity ratio. Each of these measures is an indication of how well or how poorly traffic flow is being accommodated by the freeway segment. 4 Service Measure Although speed is a major indicator of service quality to drivers, freedom to maneuver within the traffic stream and proximity to other vehicles are equally noticeable concerns. These other concerns are related to the density of the traffic stream. Furthermore, unlike speed, density increases as flow increases up to capacity, resulting in a measure of effectiveness that is sensitive to a broad range of flows. 5 Level of Service For these reasons, density is the performance measure used to define levels of service for basic freeway segments (thus referred to as the service measure). 6 Level of Service (LOS) Concept – a qualitative measure describing operational conditions within a traffic stream and their perception by drivers and/or passengers. The Highway Capacity Manual (HCM), produced by Transportation Research Board (2010), gives a range for quantifying traffic operational performance Six Level of Service: A to F 7  LOS A (Freeway) Free flow conditions Vehicles are unimpeded in their ability to maneuver within the traffic stream Incidents and breakdowns are easily absorbed 8  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 9  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 10  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 11  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 12  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 13 14  The ranges of density used to define levels of service are as follows: passenger cars per mile per lane (pc/mi/ln) 15 A Passenger Car Unit (PCU) is a method used in Transport Modelling to allow for the different vehicle types within a traffic flow to be assessed in a consistent manner. Typical factors are 1 for a car or light goods vehicle, 2 for a bus of heavy goods vehicle, 0.4 for a motorcycle and 0.2 for a pedal cycle. 16 17 18