A Comparison of Floating Car vs.Loop Detector Estimated Freeway Travel Time Delay

Floating car-and loop detector-based methods are two different types of methods frequently used to collect travel time delay information across a freeway network.Sometimes,it is necessary to use them jointly to achieve the necessary freeway network coverage,due to the high labor costs for the floating car-based method and the indispensability of sufficient network instrumentation for the loop detector-based method.For example,both floating car-and loop detector-based methods were once used in the Highway Congestion Monitoring Program in the California Department of Transportation.It is therefore necessary to evaluate whether these two types of methods estimate similarly in terms of total travel time delay.To this end,corresponding delay information estimated using both types of methods from 37 freeway segments in the Greater Sacramento Area were collected and compared.It was found that these two types of methods do not estimate similarly in terms of total segment travel time delay.The mean absolute relative difference(MARD)can be as high as 78%,especially when delay is defined using a lower reference speed,such as 56 km/h.However,in terms of total segment travel time,the loop detector and the modified floating car method estimated similarly.The MARD is 19%.It was also found that the estimation from the different methods did correlate fairly well,which provides a means of conversion when different methods are used to monitor the total delay across a freeway network.As a spin-off,it was also found that a 1.5 km spacing of loop detectors is sufficient to achieve the 19%MARD as compared with the modified floating car method in terms of total travel time estimation.

A Discussion on the Current MUTCD Standards on Traffic Control Signals for Freeway Entrance Ramps

In Section 4I.02 of the newly published Manual on Uniform Traffic Control Devices(MUTCD),standards were specified for the placement of signal heads for traffic control signals at freeway entrance ramps,or ramp meters.For simultaneous operations,i.e.when ramp control signals are operated such that green signal indications are always displayed simultaneously to all of the lanes on the ramp,a minimum of two signal faces per ramp shall face entering traffic.For staggered operations,i.e.the ramp control signal are operated such that green signal indications are not always displayed simultaneously to all of the lanes on the ramp,one signal face shall be provided over the approximate center of each separately-controlled lane.Based on a nation-wide survey,it was realized that the two standards did not fully reflect the state-of-the-practice with respect to ramp metering operations.Due to the lack of significant conflicting movements at ramp meters,for any on-ramp lane,a minimum of one overhead mounted,or one upper and one lower roadside-mounted signal face is sufficient to control the metering operations.The minimum necessary number of signal faces changes according to the total number of lanes,how the signal faces are mounted(overhead or roadside mounting),and how the meters are operated(simultaneous or staggered).Suggestions are made to revise the two standards to better reflect the current nation-wide practices,and better consider the specific operational needs for ramp meters.

The Sight Distance Issues with Retrofitted Single-Lane HOV Facilities

It is well-known that obstruction inside a highway horizontal curve will lead to impaired sight distance.Highway alignment design standards in terms of the minimum horizontal curve radius are specified to allow for adequate stopping sight distance at given design speeds.For a singlelane HOV facility,inside curve obstruction may occur no matter when the facility curves to the left(per travel direction)or right.A unique situation that calls for special attention is that the adjacent mixed-flow lane traffic,once queued,may become sight obstruction.Calculations indicated that such obstruction may govern the minimum curve radius design as long as the left shoulder is not less than 0.92 m,when the HOV lane is contiguous to the mixed-flow lanes.Such governance may necessitate design speed reduction,horizontal and cross-section design adjustment,or both.

An Evaluation of Ramp Control Signal Head Placement Based on Motorist’s Cone of Visions

Roadside mounted ramp control signals have one upper head,and one lower head in California.The upper head is intended for use by the approaching motorists,while the lower one for use by the stopped motorists.Complaints have been received regarding the visibility of these signal heads.The evaluation presented in this paper is to mathematically analyze the visibility of these signal heads on a tangent roadway alignment based on satisfactory motorists’cones of vision.Roadway design,signal placement,and driver characteristics are integrated into one analytic framework.The results indicated that the horizontal satisfactory cone of vision controls the placement design.To meet the 20 degree satisfactory horizontal cone of vision as specified by the Manual on Uniform Traffic Control Devices(MUTCD),a ramp control signal head has to be placed a minimum of 13.3 m downstream of the limit line.

Ramp Metering Status in California

The purpose of this paper is to provide an update of the major improvement in terms of ramp metering design and operations in California.These updates include ramp metering policies,ramp metering development plans,ramp metering design manual,and ramp metering and system management initiatives.

Queue Storage Design for Metered On-Ramps

Queue storage at a metered freeway on-ramp is an essential design element for metered on-ramps to prevent on-ramp queue from extending beyond on-ramps.In this paper,various existing methodologies that are used to size the queue storage at metered on-ramps were first reviewed.It was found that queue storage sized using 7%of peak hour on-ramp demand is widely accepted in practice.A limited dataset collected in California helps provide some reality check for this method.It is recommended that queue storage be recognized as an indispensable design element at metered on-ramps,and detailed sizing guidance be developed in the highway geometric design policy of the American Association of State Highway and Transportation Officials(AASHTO).