Michael P Pratt

Model for Predicting Speed Along Horizontal Curves on Two-Lane Highways

This paper describes the development and calibration of a curve speed prediction model. The model includes variables for curve radius, deflection angle, superelevation rate, and tangent speed. The model is based on the hypothesis that drivers modify their side friction demand because of a desire for both safe and efficient travel. One model component reflects a general desire by motorists to have lower side friction demand on higher speed curves. This trend likely reflects the drivers desire to maintain an acceptable margin of safety against sliding out or rolling over. A second model component reflects a willingness by drivers to tolerate slightly higher side friction demand on sharper curves in an effort to minimize the amount of speed reduction. The increase in side friction demand that a driver accepts is found to be proportional to the energy required to slow the vehicle to the curve speed.

Procedure for Developing Accident Modification Factors from Cross-Sectional Data

This paper describes a procedure for developing accident modification factors (AMFs) by using a cross-sectional study. It is recognized that AMFs are most accurately derived from controlled experiments and observational before-after studies. However, the execution of experiments and before-after studies is not always practical or feasible. The procedure described in this paper is intended to be used in this situation. The procedure is demonstrated through the development of a curve radius AMF for rural two-lane highways.

Running Time Prediction for Signalized Urban Streets

The development of a procedure for predicting the average running time along an urban street segment is described. Running time is shown to be a function of free-flow speed, running speed, residual lost time at the upstream intersection, and the volume of vehicles turning from the traffic stream. The procedure includes models for estimating free-flow speed, running speed, and residual lost time. These models were calibrated with field data from many sites collectively located in several states. The model for estimating free-flow speed includes variables for speed limit, median type, access point density, curb presence, and number of lanes. An increase in access point density was found to correspond to a reduction in free-flow speed. The model for estimating running speed includes a variable for flow rate. The effect of flow rate on speed is similar to that found on highways, such that speed decreases with an increase in flow rate, especially at high flow rates. The procedure is recommended for inclusion in the chapter on urban streets segments in the 2010 edition of the Highway Capacity Manual.

Evaluation of Operational Impacts of Installation of Centerline and Edge Line Rumble Strips

Research findings have shown that centerline rumble strips (CRSs) and shoulder rumble strips (SRSs) have significant safety benefits. SRSs have proved successful in reducing run-off-the-road crashes, and more recent research has shown that CRSs can potentially reduce head-on collisions. However, concerns about the operational effects of CRSs and SRSs remain. Specifically, how do the installations affect vehicle lateral position, passing operations, and shoulder usage? This paper documents the findings of research conducted for the Texas Department of Transportation on CRSs and edge line rumble strips (ERSs), a variation on SRSs that are placed directly on the marked edge line. Before-and-after analyses were conducted on Texas highways where CRSs or ERSs, or both, were installed. CRSs were evaluated with respect to passing operations and vehicle lateral position within the travel lanes; ERSs were evaluated for shoulder usage and vehicle lateral position during shoulder encroachments. The results of the stu...

Evaluation of Alternative Procedures for Setting Curve Advisory Speed

This paper examines procedures for setting curve advisory speeds, identifies issues associated with these procedures, and describes a proposed procedure that overcomes many of these issues. The proposed procedure consists of three elements: a criterion for setting the curve advisory speed, a method for determining this speed, and an engineering study method that confirms the need for an advisory speed plaque. One conclusion is that challenges associated with the use of the ball-bank indicator hinder achieving curve advisory speeds that are uniform in curves and consistent with driver expectation. The proposed procedure is shown to overcome these challenges.

Assessing Curve Severity and Design Consistency Using Energy- and Friction-Based Measures

Numerous published models can be used to predict curve speed based on geometric and operational characteristics like radius, superelevation rate, and approach tangent speed. Speed-based design consistency measures have also been developed to help identify which curves on a roadway are the most severe. However, the use of speed reduction alone can result in improper assessment of curve severity because drivers are more reluctant to reduce speed on roadways with higher speeds and thus accept speeds associated with higher crash risk. New measures of curve severity are suggested, based on considerations of side friction demand and kinetic energy. The increase in side friction demand above drivers’ comfort thresholds is shown to be roughly proportional to the kinetic energy reduction associated with speed reduction. Agencies can use these curve severity measures to assist in identifying curves in their jurisdictions that would most likely benefit from safety improvements.

Severity Distribution Functions for Freeway Segments

To date, the focus of modeling efforts for freeway safety has been on developing safety prediction functions and crash modification factors, with only limited consideration for crash severity distributions. As a result, relatively little is known about the safety effects of design elements such as lane width, rumble strips, and longitudinal barriers on crash severity. In some cases, countermeasures are implemented with the intent to reduce fatal crashes, but the effect of these treatments on less severe crashes is not well understood. Research was conducted to develop severity distribution functions (SDFs) to predict the proportion of crashes in each severity category as a function of roadway geometric design elements and traffic control features. The SDFs were calibrated with freeway segment data from California, Maine, and Washington State. The findings from this research show that barrier presence, increased traffic volume, increased lane width, and urban area type reduce the proportion of high-severity crashes. At the same time, the presence of rumble strips and horizontal curvature increases the proportion of high-severity crashes. These SDFs can be applied along with safety prediction functions and crash modification factors to obtain more precise estimates of the safety effects of design decisions.

Influence of area population, number of lanes, and speed limit on saturation flow rate

Research has shown that area population, the number of lanes in the lane group, and speed limit are correlated with saturation flow rate. However, the procedures in Chapter 16 of the 2000 Highway Capacity Manual (HCM) do not recognize these influences. The objective of this paper is to report the findings and conclusions from a research project that examined the influence of these and other factors on the saturation flow rate of the through lane group at several signalized intersections. An examination of several thousand headway observations indicates that saturation flow rate increases in a predictable way with an increase in population, number of lanes, or speed limit. Several saturation flow rate adjustment factors are calibrated on the basis of field data and are recommended for inclusion in the next edition of the HCM.

Analysis of Injury Severity in Crashes on Ramps and at Crossroad Ramp Terminals

Travel along a ramp can present drivers with complex alignment changes and decision points. These changes and complexities can increase the potential for conflict or crash, especially for larger vehicles. A few studies have examined the effect of variables on ramp crash frequency. However, none of these studies considered the effect of variables on crash severity distributions. Relatively little or no information is available about the safety effects of design elements on the severity of ramp crashes. In some cases, countermeasures (such as longitudinal barriers) are implemented to reduce fatal crashes, but the effect of these treatments on less severe crashes is not well understood. New research has been conducted to develop severity distribution functions (SDFs) for ramp segments and signalized and unsignalized crossroad ramp terminals for predicting the proportion of crashes in each severity category as a function of roadway geometric design elements and traffic control features. The SDFs were calibrated with data from California, Maine, and Washington State. The findings from this research show that barrier presence, the number of through lanes, area type, and ramp type influence the proportion of high-severity crashes on ramp segments. At the same time, access point frequency, left-turn operation, the presence of nonramp public street leg, and area type influence the proportion of high-severity crashes at crossroad ramp terminals. These SDFs can be applied along with safety performance functions and crash modification factors to obtain more precise estimates of the safety effects of design decisions.

Framework for Evaluating Advance-Detection Designs for High-Speed Intersection Approaches

This paper describes a framework for evaluating an advance-detection design. The framework can be used to identify the most effective configurations for a given detection design. It accounts for the detection layout, including number and position of detectors; traffic conditions, including volume and speed; and controller settings, including passage time and operation of the stop line detector after queue clearance. The framework is demonstrated by application to six advance-detection designs used in Texas. The framework is based on measures of control delay, probability of ending the green phase through max-out, and probability of providing indecision-zone protection during green-phase gap-out. The latter measure is defined as “detection coverage.” These measures are useful for quantifying the operational and safety performance of a detection design. The framework can be used (a) to select an advance-detection design for an intersection approach with a given 85th percentile traffic speed and (b) to determine the range of traffic speeds for which the detection design can provide acceptable indecision-zone coverage. This information is useful for sites where traffic speeds are not known precisely or are expected to vary over the day, week, or month. It can also be used to determine how changes in controller settings will affect the performance of the detection design.