Ahmed Al-Kaisy

Freeway Quality of Service: What Really Matters to Drivers and Passengers?

Although the concept of level of service for freeways is usually defined in terms of users’ perceptions, very few studies have sought drivers’ or passengers’ views about what is important to them. Such information is particularly important for the evaluation of extended trips on freeways as opposed to trips on a single section or segment. Such information is valuable not only for improving the Highway Capacity Manual but also for establishing appropriate criteria for assessment of intelligent transportation system proposals. The results of focus group sessions are reported in which a group of commuters discussed their views about determinants of the freeway quality of service that they experienced. Total travel time is the most important determinant for them, but a number of other aspects of the trip also matter, including safety, traveler information, and maneuverability (density). The importance of travel time is a reminder that travel is a derived demand, not something that commuters do for the pleasure of the drive.

Indicators of Performance on Two-Lane Rural Highways: Empirical Investigation

This paper presents an empirical investigation into performance indicators on two-lane rural highways. Field data from four study sites in the state of Montana were used in this investigation. Six performance indicators were investigated: average travel speed, average travel speed of passenger cars, average travel speed as a percent of free-flow speed, average travel speed of passenger cars as a percent of free-flow speed of passenger cars, percent followers, and follower density. The study examined the level of association between the selected performance indicators and major platooning variables, namely, traffic flow in the direction of travel, opposing traffic flow, percent heavy vehicles, standard deviation of free-flow speeds, and percent no-passing zones. Among all platooning variables investigated, traffic flow in the direction of travel was found to have the highest correlation with performance indicators. Other platooning variables exhibited only weak correlations with performance indicators. Also, follower density followed by percent followers exhibited the highest correlations with platooning variables.

New insights into freeway capacity at work zones : Empirical case study

A construction project in Ontario, Canada, provided the opportunity to use field data to investigate freeway capacity at long-term lane closures due to rehabilitation work. Data from two lane closures at the same construction site (eastbound and westbound) were examined. The site is located on the Gardiner Expressway in the southern part of downtown Toronto. Data were collected during 4 days, totaling around 53 h of congested traffic operations. Results showed significant variation in freeway capacity in the work zones. Despite this variation, average capacity values are reasonably close to the corresponding values provided in the Highway Capacity Manual. Four intervening variables were investigated; all exhibited significant but different effects on freeway work-zone capacity. These variables included temporal variation (which is thought to relate to driver characteristics), grade, day of week, and weather conditions. The results confirmed the pressing need for more extensive field data that will allow better identification of the effect of various control variables on work-zone capacity.

Examination of effect of driver population at freeway reconstruction zones

An investigation into the effect of the driver population factor on the capacities of long-term freeway reconstruction zones is presented. A major reconstruction project in Ontario, Canada, provided an opportunity to conduct the investigation. Comprehensive data on traffic, weather, and work activity were used. Three different analyses were conducted. These analyses compared mean capacity flows during different times of the day and days of the week to estimate the effect of the noncommuter driver population during weekdays and weekends. The study found that the effect of the driver population factor on the capacity of the reconstruction site was highly significant. On the basis of a factor of 1.0 for commuter traffic, a driver population factor of 0.93 was estimated for the afternoon peak period and a driver population factor of 0.84 was estimated for weekends. Also, the driver population factor is likely responsible for a capacity reduction on weekends compared with the capacity on weekdays. This capacity reduction was 12 percent in one direction of travel and 17 percent in the other direction. Both the driver population factors and the capacity reductions on weekends found in the study are considered conservative, given that these data were collected in April and early May and that there is a higher proportion of tourist drivers during the summer season at this site. Nonetheless, numbers are consistent with empirical observations from other studies and provide further guidance for use of the driver population factor in the 2000 Highway Capacity Manual procedures.

Estimating Performance on Two-Lane Highways: Case Study Validation of a New Methodology

This study presents an empirical investigation into the use of a new measure, percent impeded, for estimating performance on two-lane highways. Percent impeded is a point estimate of the percentage of vehicles that are impeded by slower vehicles in the traffic stream due to platooning, a common phenomenon on two-lane, two-way highways. This point measure, by definition, is closest in concept to percent-time-spent-following, a spatial measure of performance on two-lane highways that is used in the current practice yet is impractical to measure in the field. Field data were collected from two two-lane highway study sites with passing lanes in the state of Montana. Performance was examined at a single location upstream and multiple locations downstream of the passing lane at each study site. As a reference, the analyses utilized some of the currently used or suggested performance measures on two-lane highways. Overall, the percent impeded patterns and trends were consistent with the mechanism of platoon break-up and formation in the vicinity of passing lanes, a concept that is well-documented in the literature. Further, study results suggest that percent impeded has more logical and consistent sensitivity to platooning variables compared with other performance measures investigated by this study.

Weather-Responsive Signal Timing: Practical Guidelines

This paper presents a systematic investigation into the effects of adverse weather on signal operation and the potential benefits of implementing weather-responsive signal control. The investigation considered isolated and coordinated signalized intersections in urban and suburban areas under various traffic conditions. Both operational and safety analyses were conducted in this investigation. Traffic signal optimization and microscopic traffic simulation were used to perform the operational analysis with average travel time as a performance measure. The safety analysis used the adequacy of change and clearance intervals and the presence of dilemma zones as indicators of safety at signalized intersections. Study results suggest that operational benefits of weather-responsive signal control are generally significant and that the greatest benefits can be realized at coordinated corridors in town and city centers. Results also suggest that dilemma zones are most likely introduced on a particular intersection approach once the coefficient of friction between tire and pavement drops below the design value of a wet pavement. Further, a set of recommended guidelines that relate weather conditions to operational impacts and potential benefits of weather-responsive signal timing was developed and provided in this study.

Field Evaluation of Truck Weigh Station Operations

Weigh-in-Motion (WIM) systems can improve the capacity of weigh-station operations by screening trucks traveling at high speeds and requiring only trucks within a threshold of a maximum permissible weight or axle load to be weighed on more accurate static scales. The efficiency of a weigh station is highly dependent on the accuracy of the WIM screening system. This article examines the operations at the Stephens City weigh station in Virginia. The case study evaluates the accuracy of the WIM technology, in addition to the operations of the weigh station, in terms of service time (time that the truck is on static scale), queued delay, and system time. Specifically, the study demonstrated that the WIM systems did not conform to American Society for Testing and Materials (ASTM) standards for Type III WIM systems. The study also demonstrated that by incorporating a weight threshold of 96% only 16% of the trucks that were identified as possible violators actually violated the legal limits. Consequently, 84% of the trucks that were required to enter the static scale were forced to incur unnecessary delays and caused additional delays for violating and nonviolating trucks.

Measuring Performance on Two-Lane Highways: Empirical Investigation

This paper presents an empirical investigation into several performance measures for operational analysis on two-lane highways. The performance measures investigated are average travel speed, average travel speed to free-flow speed, percent followers, followers flow, followers density, percent impeded, impeded flow, and impeded density. Field data from 16 study sites in the states of Montana, Idaho, Oregon, and North Carolina representing Class 1, Class 2, and Class 3 highways were used in this study. The level of association between performance measures and some of the most important traffic variables was examined with graphical and statistical techniques. The traffic variables investigated in this study included combined flow in both directions of travel; proportion of traffic in the direction of travel, called in this study “traffic split”; percentage of heavy vehicles; and speed variance. Study results suggest that speed-related measures have weak associations with traffic variables compared with headway-related measures. Further, compound measures involving headway and traffic flow or density exhibited the highest associations with traffic variables. With regard to two-lane highway type, higher associations are exhibited at Class 1 sites compared with Class 2 and Class 3 sites. Performance measures showed the highest associations with combined flow and traffic split.

Safety Effects of Road Geometry and Roadside Features on Low-Volume Roads in Oregon

Crashes are random events and can occur at any location along a roadway. On roadways with high traffic volumes, the more frequent occurrence of crashes permits the direct identification of high-frequency crash locations with the use of historical data. On low-volume roads, crash occurrence, particularly the occurrence of crashes with fatal and serious injuries, is less frequent. There is a need to understand better the risks associated with geometric and roadside features along low-volume roadways in order to identify locations where preventive countermeasures may be employed. This paper describes the collection and analysis of a large sample of data from low-volume roads in Oregon to quantify the effects of geometric and roadside features on crash occurrence and associated risks. The effects of lane width, shoulder width, grade, side slope, fixed objects near the roadway, and horizontal and vertical curves have been quantified. For the low-volume road sample, roads with lanes less than 12 ft wide have a much higher crash risk than do roads with standard 12-ft lanes. Similarly, roads with narrow or no shoulders tend to have much higher crash rates than roads with shoulders 4 ft or 5 ft wide. Crash risk is shown to be much higher on curves with higher degrees of curvature compared with curves with smaller degrees of curvature.

Traffic Use of Rest Areas on Rural Highways

Rest areas provide the occupants of passenger vehicles and the operators of heavy vehicles an opportunity to use a restroom, walk around, stop for a meal, sleep for a while, or pause to use a cellular phone. These activities have a direct impact on several aspects of the design of rest areas, from parking to facility sizing, water needs, and wastewater generation and handling. All these components are directly influenced by one critical factor: entering traffic volumes. The study presented here used data from 44 study sites to examine the amount of traffic that used rest areas, expressed as the percentage of the main-line hourly volume that entered the rest area, as well as the effect of many underlying variables that were believed to affect the use of rest areas. The study found that the average rate of rest area use by main-line traffic for different highway categories (high- and low-volume Interstates and arterials) varied from 8.4% to 12.3%. For main-line traffic entering rest areas, the overall average rate of use was approximately 10% and the overall 85th percentile was about 15%. The study identified two peaks during the day for the percentage of main-line traffic using the rest area, but vehicular counts at rest areas showed only one peak at about midday. Given this peak demand, the midday period should be considered in the planning and design of rest area facilities. For the majority of rest areas examined, the average rate of use during the midday period varied roughly from 13% to 17% of main-line traffic.