Lidar Data-Based Method to Measure Sight Distance at Unsignalized Intersection

A gap acceptance crash is one of the most common types of multivehicle crashes at unsignalized intersections. Drivers’ gap acceptance is influenced by the intersection sight distance (ISD). Before this research, the relationship between ISD and safety (measured by crash frequency) at unsignalized intersections was not quantified. To establish this relationship, crash, traffic, and geometric data were collected from 832 two-lane minor unsignalized intersection approaches in North Carolina, Ohio, and Washington State. These intersections represent a range of volumes and major road geometrics (two-lane and four-lane undivided and divided) in urban and rural areas. The sites were selected for data collection independent of historical crash frequency. The data collection included field-measured ISD using a standardized method and an exploratory measure of ISD quality. For analysis of the data, a cross-sectional study design was used to quantify the relationship between safety and ISD. The results suggest that target crashes and target fatal and injury crashes are associated with available ISD, and ISD quality is related to safety performance. Moreover, the results suggest that the impact of ISD on target crashes and target fatal and injury crashes varies as a function of the major and minor road traffic volumes.

SummaryProper intersection sight distance can effectively lower the possibility of intersection accidents. American Association of State Highway and Transportation Officials (2011) provide a series of recommended dimensions of intersection sight triangles for uncontrolled and stop/yield‐controlled intersections. However, in reality, although the actual intersection design for unsignalized intersections satisfies the requirements of sight distance and clear sight triangle in American Association of State Highway and Transportation Officials' guideline, there are still a large number of crashes occurring at unsignalized intersections for drivers running stop/yield signs or failing to slow down. This paper presents a driving simulator study on pre‐crash at intersections under three intersection field of view (IFOV) conditions. The aim was to explore whether better IFOVs at unsignalized intersections improve their emergent collision avoidance performance under an assumption of valid intersection sight distance design. The experimental results show drivers' ability to identify potential hazards to be significantly affected by their IFOVs. As drivers' IFOV improved, drivers were more likely to choose braking actions to avoid collisions. Better IFOVs were also associated with significant increases in brake time to intersection and significant reductions in deceleration rate and crash rate, thus leading to a lower risk of traffic crash involvement. The results indicate that providing a better IFOV for drivers at intersections should be encouraged in practical applications in order to improve drivers' crash avoidance capabilities. Copyright © 2016 John Wiley & Sons, Ltd.

Sight distance and vehicle speed are closely related to traffic safety, and good sight distance not only enable driver to judge the road traffic environment correctly, determine the correct driving behavior, but also determines effective operating time of the driving behavior. This paper made an in-depth analysis to intersection sight distance and turning characteristics, and carried on a statistical analysis to relationships among stopping distance, landscape static sight distance and the difference between them with vehicle speed, which took Mudanyuan Intersection of Haidian District in Beijing as an example and used mathematical models of the landscape static sight distance and the stopping distance. From the experiments we find that intersection sight distance have a strong correlation with vehicle velocity. Intersection static sight distance and sight distance difference have positive correlations with turning right vehicle velocity, while have negative correlations with direct vehicle velocity. Besides, it can be seen by combining the existing data that the greater static sight distance and sight distance difference are, the higher the maximum speed which is allowed and enable ensure the safety of traveling and turning right vehicle velocity are, but direct vehicle speed is lower, which has certain relations with driver psychology. Direct driving in the situation of smaller static sight distance and sight distance difference, drivers will be more anxious and then they are more inclined to speed up to avoid transportation conflicts and accidents; but if static sight distance and sight distance difference are greater, drivers will control accelerator and brake calmly, and the moving velocity will be lower.

A Policy on Geometric Design of Highways and Streets, 6th Edition, (publisher: AASHTO), provides design criteria for minimum sight distances, including intersection sight distance (ISD). An understanding of the relationship between ISD and safety at stop-controlled intersections is needed, with potential applications of this knowledge to both performance-based design and substantive road safety management practices. To establish this relationship, crash, traffic, and geometric data were collected from 832 two-lane minor unsignalized intersection approaches in Ohio, and Washington. The data were analyzed using a cross-sectional study design to quantify the relationship between safety and ISD. The analyses indicated that the expected number of target crashes are associated with available ISD. Target crash frequencies increase as available ISD decreases. Results suggest that ISD is associated with expected crash frequency in a non-linear fashion. The sensitivity of the expected number of target crashes to changes in ISD is highest when ISD is shorter, and decreases as ISD increases (i.e., the safety benefit of increasing ISD from 300 to 600 ft is substantially larger than the safety benefit of increasing ISD from 1,000 to 1,300 ft). The results also suggest that the impacts of ISD on crash frequencies vary as a function of the major road two-way annual average daily traffic and the major road speed limit. The sensitivity of the expected number of crashes to changes in ISD increases as both traffic volume and speed limit increase. Crash modification functions for each of the target crash types were estimated using the regression models.

Intersection sight distance (ISD) is the distance to be provided at intersections between minor and major roads. Current AASHTO policy provides an equation and charts for the required at-grade ISD so that a driver on the minor road can depart (crossing, turning left or right) safely, even though an approaching vehicle on the major road comes into view as the stopped vehicle begins to depart. The AASHTO model is based on 2 assumptions: 1) both minor and major roads are assumed to be straight without any vertical or horizontal curvature; and 2) the intersection angle is assumed to be 90 deg. In many practical situations, however, sight distance is required to be checked for an existing or proposed 3D intersection alignment where vertical curves (crest or sag) and horizontal curves overlap. This paper presents a new mathematical model for the analysis of stop-controlled ISD on 3D highway alignments that allows the major road to have vertical and horizontal curves with skewed angle, and the minor road to have a longitudinal grade. Design aids are developed to determine the available ISD for different geometric alignment variables (e.g., radius of horizontal curve, lane width, number of lanes, and vertical curve parameters). Application of the methodology is demonstrated and discussed using numerical examples.

Intersection sight distance analysis and guidelines

Intersection sight distance (ISD) requirements, currently designed for driver-operated vehicles (DVs), will be affected once automated vehicles (AVs) enter the driving environment. This paper examines the ISD for intersections with a yield control on a minor road in a mixed DV-AV environment. Five potential conflict types with different ISD requirements are modeled as a minor-road vehicle proceeds to cross the intersection, turns right, or turns left. Furthermore, different models are developed for each conflict type depending on the vehicle types on the minor and major roads. These models, along with the intersection geometry, establish the system demand and supply models for ISD reliability analysis. A surrogate safety measure is developed and used to measure ISD non-compliance and is denoted by the probability of unresolved conflicts (PUC). The models are applied to a case study intersection, where PUC values are estimated using Monte Carlo Simulation and compared to an established target value relating to the DV-only traffic of 0.00674. The results show that AV-related traffic has higher overall PUC values than those of DV-only traffic. A corrective measure, reducing the AV speed limit on the minor-road approaches by 3 to 4 km/h, decreases the overall PUC to values below those of the target PUC.

The intersection sight distance (ISD) design presented by AASHTO is based on extreme values of the component design variables such as design speed, perception–reaction time (a high percentile), and friction coefficient (a low percentile). A reliability method is presented, based on AASHTO, that does not rely on extreme values but instead considers the moments (mean and variance) of the probability distribution of each random variable. The method also accounts for correlations among the component random variables. In Cases I and II of AASHTO, the variations of the sight distance along both legs of the intersection are considered for both design and evaluation. For evaluation (involving an exiting obstruction), these variations are combined into a single variable that determines whether the corresponding sight line is obstructed. In Case III, only the sight distance leg along the major road has variations. The proposed method is straightforward and involves simple, closed-form mathematics for calculating sight distance and associated reliability. Sensitivity of ISD to various design variables is examined. ISD reliability-based values for various cases are presented from data reported in the literature, and results are compared with current AASHTO design values.

Many traffic accidents are caused by unforeseen and unexpected events in a site that was hidden from the driver's eyes. Road design parameters determining required visibility are based on relationships formulated decades ago. It is worth reviewing them from time to time in the light of technological developments. In this paper, sight distances for stopping and crossing situations are studied in relation to the assumed visual abilities of autonomous vehicles. Current sight distance requirements at unsignalized intersections are based among others on speeds on the major road and on ac-cepted gaps by human drivers entering or crossing from the minor road. Since these requirements vary from country to country, regulations and sight terms of a few selected countries are compared in this study. From the comparison it is remarkable that although the two concepts, i.e. gap acceptance on the minor road and stopping on the major road have different backgrounds, but their outcome in terms of required sight distances are similar. Both distances are depending on speed on the major road: gap sight distances show a linear, while stopping sight distances a parabolic function. In general, European SSD values are quite similar to each other. However, the US and Australian guidelines based on gap acceptance criteria recommend higher sight distances. Human capabilities and limitations are considered in sight field requirements. Autonomous vehicles survey their environment with sensors which are different from the human vision in terms of identifying objects, estimating distances or speeds of other vehicles. This paper compares current sight field requirements based on conventional vehicles and those required for autonomous vehicles. Visibility requirements were defined by three vision indicators: distance, angle of view and resolution abilities of autonomous cars and human drivers. These indicators were calculated separately for autonomous vehicles and human drivers for various speeds on the main road and for intersections with 90° and 60° angles. It was shown that the required sight distances are 10 to 40 meters shorter for autonomous vehicles than for conventional ones.

The American Association of State Highway and Transportation Officials (AASHTO) model of calculating sight distance at intersections without control (Case I) has been modified to explicitly incorporate the design speeds of both intersecting roads in computing the required sight distance for each road. It is shown in this paper that using design speeds of both roads in the modified model will result in an inadequate sight distance for most vehicles. To address this issue, this paper presents a revised model that is based on a small percentile speed of the intersecting road (such as the 1 or 5 percentile). Using the revised model, design requirements of intersection sight distance (ISD) are established for passenger cars and trucks. The results show that the practice of using design speed in ISD analysis may result in significant underestimation of ISD requirements, particularly when the speed of the approaching vehicle on the intersecting road is extremely low. Furthermore, the amount of underestimation increases with an increase in the difference between the design speeds of the intersecting roads. For intersections with sight-distance restrictions, the standard solution is to reduce vehicle speed. However, it is interesting that, for some obstruction locations, it is necessary to increase the speed to satisfy sight-distance needs. An analytical method and a worksheet for finding the required (reduced or increased) speed are presented.

Intersection sight distance(ISD) is an important design element. Each intersection has a potential for several different types of vehicular conflicts that can be greatly reduced through the provision of proper sight distance. Current guidelines do not adequately address sight distance requirements for intersections located on horizontal curves alone or horizontal curves combined with vertical alignments. In many practical situations, however, sight distance is required to be checked for an existing or proposed three-dimensional(3D) intersection alignments. In this thesis, models were developed to check sight (2001) were considered on 3D alignment: (1)Departure from stop-control minor-road and (2) Left-turns from major-road. For stop-control intersections, several cases were addressed. These include Case 1(a): Intersection and approaching vehicle (object) lie on the curve, Case 2: Intersection lies on the tangent and object lies on the curve. For both cases (1) and (2), obstruction may lie inside or outside the horizontal curve and the intersection and object can be anywhere with respect to the vertical alignment. In many practical situations, however, sight distance is required to be checked for an existing or proposed three-dimensional(3D) intersection alignments. In this thesis, models were developed to check sight (2001) were considered on 3D alignment: (1)Departure from stop-control minor-road and (2) Left-turns from major-road. For stop-control intersections, several cases were addressed. These include Case 1(a): Intersection and approaching vehicle (object) lie on the curve, Case 2: Intersection lies on the tangent and object lies on the curve. For both cases (1) and (2), obstruction may lie inside or outside the horizontal curve and the intersection and object can be anywhere with respect to the vertical alignment. Design aids for required minimum lateral clearance (from the minor and major roads) are presented for different radii of intersections located on horizontal curves, guidelines are presented for offsetting opposing left-turn lanes to provide unobstructed required sight distance. Applications of the methodologies are illustrated using numerical examples.

Intersection sight distance(ISD) is an important design element. Each intersection has a potential for several different types of vehicular conflicts that can be greatly reduced through the provision of proper sight distance. Current guidelines do not adequately address sight distance requirements for intersections located on horizontal curves alone or horizontal curves combined with vertical alignments. In many practical situations, however, sight distance is required to be checked for an existing or proposed three-dimensional(3D) intersection alignments. In this thesis, models were developed to check sight (2001) were considered on 3D alignment: (1)Departure from stop-control minor-road and (2) Left-turns from major-road. For stop-control intersections, several cases were addressed. These include Case 1(a): Intersection and approaching vehicle (object) lie on the curve, Case 2: Intersection lies on the tangent and object lies on the curve. For both cases (1) and (2), obstruction may lie inside or outside the horizontal curve and the intersection and object can be anywhere with respect to the vertical alignment. In many practical situations, however, sight distance is required to be checked for an existing or proposed three-dimensional(3D) intersection alignments. In this thesis, models were developed to check sight (2001) were considered on 3D alignment: (1)Departure from stop-control minor-road and (2) Left-turns from major-road. For stop-control intersections, several cases were addressed. These include Case 1(a): Intersection and approaching vehicle (object) lie on the curve, Case 2: Intersection lies on the tangent and object lies on the curve. For both cases (1) and (2), obstruction may lie inside or outside the horizontal curve and the intersection and object can be anywhere with respect to the vertical alignment. Design aids for required minimum lateral clearance (from the minor and major roads) are presented for different radii of intersections located on horizontal curves, guidelines are presented for offsetting opposing left-turn lanes to provide unobstructed required sight distance. Applications of the methodologies are illustrated using numerical examples.

Estimation of passenger car unit (PCU) is a vital component of any traffic study conducted in developing countries owing to the heterogeneity in the vehicular traffic. Past studies on PCU factors are majorly limited to intercity and urban roads. However, PCU factors for different types of vehicles on unsignalized intersections have not been investigated so well. This paper presents three methods of estimating PCU factors for different types of vehicles on unsignalized intersections under highly heterogeneous traffic conditions. The first method is based on the occupancy time of a vehicle while clearing the intersection. The second method is based on the capacity of a priority movement estimated in terms of different vehicle categories. Queue clearance rate is used as the basis for PCU estimation in the third method. Data were collected at two unsignalized intersection in semi-urban area of two cities of India and PCU for different types of vehicles is determined using the three methods. The resulting values of PCU factors were found to be logical and are representatives of the actual field conditions.

Many Highway Accidents Occur At Intersections That Are Not Controlled By Traffic Signals. Intersecting Roadways, Driveways, And Entrances To Shopping Malls Are Only A Few Examples Of Where Crashes Occur. In Some Cases Tort Lawsuits Are Brought Against The Highway Agency Claiming That There Was A Lack Of Sight Distance At The Intersection. Unfortunately, Many Engineers And Accident Reconstruction Professionals Do Not Understand Intersection Sight Distance Requirements And Guidelines. This Confusion Often Leads To Needless Litigation. This Paper Will Review The Highway Design Guidelines (Aashto) Which Are Used To Evaluate Intersection Sight Distance For Both Turning And Straight Through Traffic Movements. It Will Explain The Rationale For The Criteria And How It Should Be Applied For Accident Analysis. The Drivers Responsibility, Under The Vehicle Code, At An Intersection Is Reviewed. A Case Study Will Be Used As An Example To Explain The Procedures For Field Measurements At An Intersection.

The American Association of State Highway and Transportation Officials presents guidelines for intersection sight distance (ISD) based on extreme values of the component design variables such as speed and time gap. The guidelines for a stopped vehicle at a stop-control intersection on a minor road assume that the component design variables are deterministic. This paper presents a reliability method that considers the design variables as random. The method uses the means and variances of the probability distributions of the random variables and accounts for their correlations. A safety margin is defined as the difference between available and required ISD. Relationships for the mean and standard deviation of the safety margin are developed based on first-order second-moment analysis. The proposed method is useful for ISD design of a new intersection or redesign of an existing intersection for a desired reliability level. For single-unit trucks, the obstruction clearances from the minor- and major-road pavement edges are approximately 10% higher than those for passenger cars. The proposed method should be of interest to highway engineers involved in road safety and management.