The paper documents studies of posted speed limit (PSL) changes on the free-flow speed on urban roads. Before and after field measurements were conducted, changing the existing PSL from 50 to 40 or $60\mathrm{km}/\mathrm{h}$. The analysis was conducted on the mean free-flow speed difference and speed variability. The data collected were also used for multiple regression analysis, including PSL changes and selected self-explaining road characteristics. The results showed that a decreased PSL caused a small ($1.6\mathrm{km}/\mathrm{h}$) but significant reduction in the mean free-flow speed and speed variance, which might lead to a 10% reduction of severe injury accidents. Furthermore, the PSL reduction had a larger impact on faster drivers and higher road network classes. Conversely, an increased PSL resulted in a $2.6\mathrm{km}/\mathrm{h}$ increase in the mean free-flow speed but no change on speed variability. The regression results indicated that the free-flow speed was heavily influenced by road characteristics, such as carriageway width, road environments, and the presence of on-street parking and sidewalks. Arterial roads presented the largest impact. The PSL had a relatively small impact.

The introduction of the passenger car unit (PCU) concept by the 1965 edition of the Highway Capacity Manual (HCM) enabled researchers around the world to deal with mixed traffic flow. Though some researchers have tried to show the dynamic nature of PCU factors, most of the studies in the world, as well as the latest standards being followed, still provide static PCU values for various types of vehicle categories. The present study demonstrates the dynamic nature of PCU factors on two-lane intercity highways under highly heterogeneous traffic composition. They are estimated based on speed and size of vehicle type in the traffic stream with respect to a standard passenger car. Simultaneous equations are formulated to calculate the speeds of different types of vehicles for a given traffic composition and traffic volume on the road, which lead to the determination of PCU factors. While many plots are made to describe the effect of traffic composition and volume on PCU of a vehicle type, it is felt that they are not enough to explain the complete variation. Therefore, the concept of a stream equivalency factor is suggested to convert a heterogeneous traffic stream into a homogeneous stream consisting of passenger cars. A simple linear equation is developed using data from three sites to calculate this factor for a known traffic volume and proportional composition of a traffic stream. Field data from a fourth site are used to validate the proposed model. The proposed model is quite simple to use and would avoid estimation of PCU factors for individual types of vehicles on a two-lane intercity highway.

Tolling has been one of the most effective countermeasures against traffic congestion in metropolitan areas. Prior to the implementation of a tolling strategy, it was necessary to systematically evaluate the toll impacts on the transportation network. Microscopic simulation has been widely utilized as an evaluation tool in many traffic operation studies and practice. Most simulation tools on today’s market, however, do not carry the functionality needed for customized toll-strategy assessment with the built-in modules. With the booming adoption of congestion pricing nationwide, it is highly desirable to enable simulation-based evaluation of various tolling strategies through external module development for widely-used simulation tools. This paper presents an innovative method for developing highly customized tolling strategy simulation through external routing modules compatible with a widely-used simulation tool. Using a publicly-available module, customized tolling schemes and routing algorithms can be implemented in the simulation tool. The method is demonstrated using the SR-520 toll bridge in Puget Sound as a test. The simulation results indicate that the developed module can effectively assess the impact of a variety of tolling schemes by determining travelers’ routing decisions in real-time. The network-wide tolling impacts on link volume, travel speed, and emissions are evaluated in this study. The proposed method facilitates the understanding of key factors that would affect travelers’ route choice and has great potential in tolling-strategy evaluation for transportation networks.

Traffic safety policy defines plans that are expressed using numerical values for the planned level of improvement. The directing or apportioning of efforts represents the initial step of planned improvements. If a road is observed as a system of sections, the sections should be prioritized, selected, and investigated according to precise enhancement requests. A temporal approach to road safety analysis enables the application of reliability reallocation theory. The individual application of certain reliability reallocation methods may yield results that are unusable for a road authority in some situations. This paper describes a method that is based on a judicious combination of acceptable features of two reliability reallocation methods [the aeronautical radio incorporated (ARINC) and the minimum effort algorithm] in different iterations. This method enables the possibility of setting a precisely expressed goal with a minimal set of input values to obtain quality and applicable results for a decision maker. The presented approach represents an improvement in the field of road safety decision-making support and is transferable to other areas of transportation engineering and effort allocation.

Threats of wind gusts on the safety of road vehicles have been reported around the world, and extensive studies have been carried out to find the accident speeds of road vehicles under a sudden crosswind. However, most previous studies assume that the angular displacements of the vehicle are small and the mass moments of the vehicle remain unchanged during the overturning of the vehicle, yielding a quasi-linear safety analysis of a road-vehicle under a sudden crosswind. This study aims at a nonlinear safety analysis of a running road vehicle under a sudden crosswind by establishing nonlinear equations of motion of a wind-vehicle system. The assumptions adopted in the previous studies are no longer required, and tires can lose or resume contact with the ground. Wind loads on the vehicle are updated with the consideration of the time-varying attitude of the vehicle. The numerical example shows that the proposed nonlinear analysis procedure is more rational than the currently used approaches for progressive instability and the accident vehicle speed of road vehicles.

To increase freight efficiency, trucks are allowed to transport heavier loads without adding extra axles. The impact of this practice on infrastructure damage needs to be evaluated. This involves evaluating the fatigue life of asphalt pavements under multiple axle configurations. In mechanistic pavement design, the asphalt thickness is generally determined using a fatigue model calibrated in the laboratory by applying a continuous sinusoidal signal with different strain levels. The classical fatigue relationship gives fatigue life as a function of the strain level. However, results from literature show the influence of the loading wave shape on the fatigue life of bituminous mixtures. Real loading signals under multiple axle configurations are more complex in shape than sinusoidal pulses. Moreover, real strain signals have different shapes in the longitudinal and transverse directions. This paper presents an approach using a multilinear fatigue model that takes into account the shape of the loading signal coupled with a viscoelastic pavement model. Using this approach, a parametric study is made to evaluate the effect of loading variables on fatigue life, as well as comparisons between the aggressiveness of different truck configurations including European Modular Systems (EMS).

The present study analyzes the effectiveness of preventive maintenance (PM) treatments of flexible pavements utilizing data obtained from the specific pavement study 3 (SPS-3) of the long-term pavement performance (LTPP) program online database. The PM treatments included in this study are thin hot mix asphalt (HMA) overlay, chip seal, slurry seal, and crack seal. International roughness index (IRI), rutting, fatigue cracking, longitudinal cracking, and transverse cracking were selected as indicators of pavement performance. Initial roughness change, long-term roughness change, and weighted distresses are used as measures of effectiveness. The logistic regression model is employed to investigate the influence of pretreatment roughness condition, pretreatment surface condition, and several other factors on the performance of PM treatments. Results from this study indicate that only thin overlay and chip seal are effective in retarding fatigue cracking. Thin overlay was also the only treatment effective in slowing long-term roughness increasing, rutting, longitudinal cracking, and transverse cracking. Sections with poor surface conditions deteriorate faster in terms of long-term roughness and longitudinal cracking. It is also found that soil type of subgrade, traffic volume, and structural capacities of sections exhibited significant influence on the effectiveness of PM treatments.

Ramp metering has been proven as an effective freeway-management strategy; however, one of the critical challenges in design and operation of ramp metering is the lack of up-to-date design guidance for acceleration lanes. This paper aims to investigate the actual acceleration characteristics for metered on-ramps with various geometric features and also figure out speed profiles to guide the design of acceleration-lane length. A piecewise-constant acceleration model, which assumes vehicles making uniformly accelerated motion within each short space or time interval, was employed to model vehicle acceleration characteristics. The approach of using distance versus speed profiles for acceleration-length design was also introduced. Results show that acceleration rate at metered on-ramps is not constant; vehicles usually accelerate at a higher acceleration rate when speed is lower and vice versa. It was observed that taper ramps usually produce higher acceleration rates than ramps with an auxiliary lane, and existing acceleration length would also affect drivers’ acceleration behavior. Finally, this study pointed out that the acceleration performance data documented in the AASHTO Green Book, which are now widely employed by several state DOTs in the United States as acceleration length design guidance for metered on-ramps, is approximately 1.7 times greater than the value measured at typical taper-type metered on-ramp in California.