Kay Fitzpatrick

Another Look at Pedestrian Walking Speed

Pedestrians have a wide range of needs and abilities. FHWAs Manual on Uniform Traffic Control Devices for Streets and Highways includes a walking speed of 4.0 ft/s (1.2 m/s) for calculating pedestrian clearance intervals for traffic signals. It also includes a comment that where pedestrians who walk slower than normal, or pedestrians who use wheelchairs, routinely use the crosswalk, a walking speed of less than 4.0 ft/s should be considered in determining the pedestrian clearance times. A 2005 TCRP-NCHRP study found a 15th percentile walking speed for young pedestrians of 3.77 ft/s (1.15 m/s) and a 15th percentile walking speed for older pedestrians of 3.03 ft/s (0.92 m/s). The study also determined that there is a statistical difference in walking speeds between older (older than 60 years) and younger (60 years and younger) pedestrians. Using population projections and the 15th percentile walking speeds for each population group more than 15 years old, the proportionally weighted 15th percentile walking...

Safety Effectiveness of HAWK Pedestrian Treatment

The high-intensity activated crosswalk (HAWK) device is a pedestrian-activated beacon located on the roadside and on mast arms over the major approaches to an intersection. It was created in Tucson, Arizona, and at the time of this study was used at more than 60 locations. The HAWK head consists of two red lenses over a single yellow lens. It shows a red indication to the motorists when activated, which creates gaps for pedestrians to use to cross the major roadway. A before-and-after study of the safety performance of HAWK devices is documented. The evaluations used an empirical Bayes method to compare the crash prediction for the after period had the treatment not been applied with the observed crash frequency for the after period with the treatment installed. Crash types examined included total, severe, and pedestrian crashes. The evaluation used data for 21 HAWK sites and 102 reference sites and found the following changes in crashes after the HAWK beacon was installed: between 13% and 29% reduction in all crashes and approximately 50% reduction in pedestrian crashes.

Accident Modification Factors for Medians on Freeways and Multilane Rural Highways in Texas

With the growing demand for safer streets and highways, state and national transportation agencies are investigating the relationships between roadway characteristics and crashes. The objective of this study was to develop accident modification factors (AMFs) for median characteristics on urban and rural freeways and on rural multilane highways. Data available for use in the evaluation included 458 mi of with-barrier segments (primarily urban, with some rural sites), 359 mi of urban without-barrier segments, and 436 mi of rural without-barrier segments. A series of negative binomial regression models was used to determine the effects of independent variables on crashes. Variables considered in developing the base models included average daily traffic, left-shoulder width, barrier offset, median (with shoulder) width, and pole density. Crashes were examined in relation to median crashes for 5 years (1997 to 2001). An AMF represents the change in safety when a particular geometric design element changes in size from one value to another. In this project, the AMFs were estimated directly from the coefficients of the models. This approach for AMF development assumes that (a) each AMF is independent because the model parameters are assumed to be independent, and (b) the change in crash frequency is exponential. AMF equations were developed for urban and rural medians with rigid barriers, urban medians without barriers, and rural medians without barriers.

Operation and Safety of Right-Turn Lane Designs

The objectives of this research by the Texas Department of Transportation were to determine the variables that affect the speeds of free-flow turning vehicles in an exclusive right-turn lane and explore the safety experience of different right-turn lane designs. The evaluations found that the variables affecting the turning speed at an exclusive right-turn lane include type of channelization present (either lane line or raised island), lane length, and corner radius. Variables that affect the turning speed at an exclusive right-turn lane with island design include (a) radius, lane length, and island size at the beginning of the turn and (b) corner radius, lane length, and turning-roadway width near the middle of the turn. Researchers for a Georgia study concluded that treatments that had the highest number of crashes were right-turn lanes with raised islands. This type of intersection had the second highest number of crashes of the treatments evaluated in Texas. In both studies, the shared through with r...

Potential Updates to 2004 Green Book's Acceleration Lengths for Entrance Terminals

AASHTOs Policy on Geometric Design of Highways and Streets (Green Book) notes that drivers entering a highway from a turning roadway accelerate until the desired highway speed is reached. Because the change in speed is usually substantial, provision is made for acceleration to be accomplished on an auxiliary lane, called an acceleration lane, to minimize interference with through traffic and to reduce crash potential. The 2004 Green Book contains acceleration lane lengths. The procedure identified to reproduce these values assumed running speed for the limited-access highway and the ramp along with acceleration rates from 1930s studies. Potential acceleration length values were then calculated by (a) updating the assumptions within the identified procedure and (b) using spreadsheets that can generate second-to-second acceleration. Findings from recent studies were compared with the existing Green Book values and the calculated suggested acceleration lengths. The suggested lengths determined in this paper, which are based on more realistic speed assumptions, more current acceleration lengths, and findings from recent research, are longer than the values in the Green Book. The paper recommends that additional research be done on acceleration lengths to determine whether the Green Book values should be increased.

Using the Rural Two-Lane Highway Draft Prototype Chapter

The TRB Highway Safety Manual Task Force is providing direction and oversight in the preparation of a new document to be called the Highway Safety Manual (HSM). The complete version of the HSM is expected to be released to the public in 2007. A draft prototype chapter (DPC) for rural two-lane highways is available for review. A Texas Department of Transportation project explored the use of the DPC by applying the methodology to two case studies and exploring methods to develop calibration factors. The lessons learned from the effort can be used to improve the DPC along with other chapters in development. Additional guidance is needed on (a) how to identify segments, (b) acceptable minimum segment lengths, (c) how best to combine the predictions for intersections with those for tangents and curves, (d) the key variables within the procedures and appropriate assumptions when data for those variables are not available, (e) the confidence in the approach for individual roadway segments as opposed to a roadway...

Detection Distances to Crosswalk Markings: Transverse Lines, Continental Markings, and Bar Pairs

The objective of this study was to investigate the relative daytime and nighttime visibility of three crosswalk marking patterns (transverse lines, continental markings, and bar pairs). In general, this study collected information on the distance from the crosswalk when the participant verbally indicated its presence. The 78 participants were about evenly divided between male and female groups and between younger (<55 years) and older (≥55 years) groups. The study was conducted with instrumented vehicles on an open road route on the Texas A&M University campus during November 2009. Data were collected during two periods: daytime (sunny and clear or partly cloudy) and nighttime (street lighting on). Markings were installed for this study at nine midblock locations. The detection distances for bar pairs and continental markings were similar, and they were statistically longer than the detection distances for the transverse markings, both day and night.

Potential Accident Modification Factors for Driveway Density on Rural Highways: From Texas Data

Accident modification factors (AMFs) for driveway density can be described as the incremental effects of driveway density on safety. The objective of this study was to develop AMFs for driveways on rural highways in Texas. Available for evaluation were 2,354 mi of rural two-lane highways and 402 centerline miles of rural four-lane highways. From a review of the data, it is recommended that the assumed base condition for driveway density be three driveways per mile. Negative binomial regression was used to determine the effects of independent variables on crashes. Driveway and segment crashes for 3 years (1999 to 2001) were used in the evaluations. AMF equations that consider driveway density for the segment were developed for both rural two-lane and rural four-lane highways.

Potential Changes to 2004 Green Book's Adjustment Factors for Entrance and Exit Terminals

AASHTOs Policy on Geometric Design of Highways and Streets (Green Book) notes that acceleration and deceleration lengths should be adjusted on graded ramps. Factors are available in the 2004 Green Book to adjust acceleration lane and deceleration lane lengths for grades of 3% to 6%. The source of the adjustment factors was provided in the 1954 Policies on Geometric Highway Design, in which they first appeared as being based on applying “principles of mechanics to rates of speed change for level grades.” Reviews of this and other documents did not reveal a procedure for determining adjustment factors. A potential source for an adjustment factor for entrance terminals is the calculation of the distance needed to accelerate from one speed to another on different grades by means of vehicle performance equations available in the literature. Potential acceleration length adjustment factors were developed and presented in this paper. The 2004 Green Book provides equations to calculate stopping sight distances for different grades. This methodology was applied to the equations used to calculate deceleration lengths so as to determine deceleration lengths for different grades. The ratio of the deceleration length on a grade to the deceleration length on a level surface formed the basis for adjustment factors for deceleration. Actual performance of vehicles on grades and on a level surface should be measured and compared with the suggested adjustment factors to determine the accuracy of those factors.

Analysis of Platoon Impacts on Left-Turn Delay at Unsignalized Intersections

This research aimed to develop a methodology for analyzing the platoon impacts on major-street left-turn (MSLT) delay at two-way stop-controlled intersections. The main idea was to use a microscopic simulation tool to simulate different platoon scenarios in opposing through traffic and then to apply regression models to capture the impacts of platoons on the delay of MSLT. Two platoon variables were adopted as a simplification of the complex platoon scenarios to make analysis of the platoon effects on MSLT delay practical. The first two steps were to build simulation models for real-world unsignalized intersections and simulate scenarios with a combination of various factors related to platoons in VISSIM simulation. The models were calibrated with field data before simulation started. The next step was to define, derive, and calibrate two platoon variables for describing the duration and intensity of platoon arrivals in the opposing through traffic. This process effectively simplified the large combination of various factors. Finally, the two platoon variables and their relationship with the change factor of MSLT delay were modeled with regression tools. A relationship between the two variables and the delay change factor was established. The relationship indicated a positive effect by upstream platoons on MSLT delay and made it possible to quantify the impacts. The findings in this research could be used for research on platoon or signal impacts on left-turn treatments.