Free-Flow Speed Model Based on Portuguese Roadway Design Features for Two-Lane Highways

Abstract

Speed is a key performance measure in economic and environrnental analyses of two-lane highways. Speed, combined with the percentage of time spent following, is also used in the assessment of levei of service.Under free-ftow conditions, the circulation of a given vehicle is not con- Speed is a major factor in the assessment of road performance. Depending on the functional classification of a given road, a design speed is established, and engineers define the geometric features of the road to ensure that drivers can, in normal traffic conditions, achieve the expected average travei speed to reach their destinations on time. Furthermore, an operating speed may be estimated for performance evaluation during the road operation period. Real environment speed measurements are required, and the operating speed is typically associated with the 85th percentile of the observed speed distribution (1). Speed is usually recognized by road planners, designers, and users as an important measure for the evaluation ofthe levei of service, speed limit defínition, design consistency and safety analyses, and other essential studies.Recognizing the role of speed in road performance evaluation, the Highway Capacity Manual 2010 (HCM) (2) recommends speed as the most appropriate concept for use in the economic DC, 2013, pp. 12-18. DOI: 10.3141/2348 and environmental analyses of two-Iane highways, including the assessment of the impact on air quality and noise leveI. ln addition, the HCM methodology to assess the levei of service of this type of road also uses the average travei speed as an input, which in tum depends on the free-flow speed (FFS) and on the traffic volume. ln other words, the average travei speed adds the effects of the delays caused by the remaining traffic to the FFS. The FFS reflects the drivers' response to the road's geometric and environmental features because drivers are not affected by the presence of other vehicles. The definition of the FFS proposed by the HCM is similar to the definition of operating speed given by the AASHTO Green Book (1). However, operating speed may also be affected by drivers' perception of risk, speed limit and enforcement, general driving practices and culture, and vehicle technology. For this reason, speed prediction models have been developed in different regions worldwide and in different time periods.Reference manuals and national guides for road design usually define operating speed for a road section rather than for specific design elements. The HCM (2) establishes a base FFS as the speed observed for roads presenting no access points and lane and shoulder widths equal to or greater than 3.6 m and 1.8 m, respectively. The HCM also provides an FFS estimation model, taking into account speed reductions to the base FFS caused by smaller cross sections and higher densities of access points.The AASHTO Green Book (1) also provides recommendations for design and operating speeds. Different design speeds are suggested according to the functional classification intended for a planned ro...

Estimating Percentile Speeds from Maximum Operating Speed Frontier

“…The database used to test the new model formulation is the same as that previously used to model the 85th percentile speed for Portuguese two-lane highways (9). The case study road sections belong to five roads in northem Portugal: N-14, N-101, N-105-2, N-206, and N-222.…”

“…In a previous operating speed study (9), the authors considered that the effects of road geometrics on speed are not cumu1ative but are dependent on the order of magnitude of practiced speeds. To comp1y with the previous assumption, the deterrninistic speed frontier takes the following exponential form: (2) where Vrnax, = maximum operating speed in e1ementj, Xjk = geometric feature k of road e1ementj, and o,~k = regression coefficients.…”

“…In a previous research study by Lobo et al, the authors presented a free-fíow speed exponential model for curves and tangents based on the 85th percentile speed observed at selected sites on Portuguese two-lane highways (9). This model already addresses some of the concems raised in Transportation Research Circular E-CI5I (1), namely, the independence of the effects of speed predictors assumed by the linear regression models and the relati vely few speed models developed for tangents.…”

Flexible Stochastic Frontier Approach to Predict Spot Speed in Two-Lane Highways

“…Compared to the authors' previous research (Lobo et al 2013;Lobo et al 2014), the database was enriched in this study with data from two recently built IP/IC roads. These roads are representative of the two-lane IP/IC roads existing in Portugal featuring overall design speeds of 80 to 90 km/h.…”

“…This work builds on the authors' previous studies on operating speed modeling (Lobo et al 2013;Lobo et al 2014), bringing together the major accomplishments of those studies while enlarging their scope and applicability. The OSFM formulation was developed and successfully tested in Lobo et al (2014).…”