233.1 At-Grade Intersections - Driveways and Entrances

Forms

Design Exception Information Form

233.1.1 Introduction

Each intersection has the potential for several different types of vehicular conflicts. The possibility of these conflicts actually occurring can be greatly reduced through the provision of proper geometry, sight distance and appropriate traffic control. The avoidance of conflicts and the efficiency of traffic operations still depend on the judgment, capabilities, and response of each driver.

Stopping sight distance is provided continuously along each roadway so drivers have a view of the roadway ahead that is sufficient to allow drivers to stop. The provision of stopping sight distance at all locations along each roadway, including intersection approaches, is fundamental to intersection operation.

Sight distance is provided at intersections to allow drivers to perceive the presence of potentially conflicting vehicles. This must occur in sufficient time for a motorist to stop or adjust their speed, as appropriate, to avoid colliding in the intersection. The methods for determining the sight distances needed by drivers approaching intersections are based on the same principles as stopping sight distances, but incorporate modified assumptions based on observed driver behavior at intersections.

The driver of a vehicle approaching an intersection must have an unobstructed view of the entire intersection, including any traffic control devices, and sufficient lengths along the intersecting roadway to permit the driver to anticipate and avoid collisions. The sight distance needed under various assumptions of physical conditions and driver behavior is directly related to vehicle speeds and to the resultant distances traversed during perception-reaction time and braking.

Sight distance is also provided at intersections to allow the drivers of stopped vehicles sufficient view of the intersecting roadway to decide when to enter or cross it. If the available sight distance for an entering or crossing vehicle is at least equal to the appropriate stopping sight distance for the major road, then drivers have sufficient sight distance to anticipate and avoid collisions. However, in some cases, this may require a major-road vehicle to stop or slow to accommodate the maneuver by the minor-road vehicle. To enhance traffic operations, intersection sight distances that exceed stopping sight distances are desirable along the major road.

233.1.1.1 Intersection Sight Distance Criteria

The minimum sight distance at driveways and entrances, measured along the centerline of the main road, is equal to the Design Entering Sight Distance based on the operating speed of the main road. This distance is based on a 3.5 ft. (1070 mm) height of eye and 2.0 ft. (600 mm) height of object. The minimum sight distance at all grade intersections measured along the minor road, is the stopping sight distance based on the operating speed of the minor road. Exceptions are sometimes necessary where the minor road is being improved to intersect a more important road. Where the more important road is being improved, the minor road is located or improved as required to obtain the required sight distance. If the operating speed of the minor road is not known, a speed of 30 mph (50 km/h) is used. Grades are determined and access points are located to provide the required sight distance as well as achieve the necessary spacing requirements contained in the Access Management Guidelines. Minimum values for stopping sight distance are described in 230.2.1.5. Minimum Entering Sight Distance is based on a stopped passenger vehicle turning left onto a two-lane roadway with no median and grades of 3% or less. Design Entering Sight Distance is based on a stopped combination truck turning left onto a two-lane roadway with no median and grade of 3% or less. These sight distance requirements insure that intersections and access points are visible to approaching traffic.

233.1.1.2 Roadway Approaches and Driveways

The location and design of approaches and driveways affect the safety and traffic handling capacity of the highway. Therefore, the location and design of each feature receives careful consideration. Each driveway and approach is designed with appropriate width and radius that will permit entry of the design vehicle without encroachment on opposing traffic lanes. MoDOT’s Access Management Guidelines will be used to determine the number, the location and the spacing requirements for approaches and driveways.

Direct access is not allowed to the through lanes on freeways since they are designed with full control of access. Access Management Guidelines are used to determine the number of access points provided on all other facilities (i.e. controlled access and normal right of way).

233.1.1.3 Design Criteria

Driveways are designed in accordance with the details shown on the standard plans. The grade controls are followed if at all practicable. The Access Management Guide (Article 940.16) defines driveway types and basic design criteria. Pavement type and thickness are contained in 940.16.10.