Module 6 : Traffic Intersection Control

Lecture 33 : Grade Separated Intersection

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	Design Components Acceleration Lane An acceleration lane is defined as extra pavement, of constant or variable width, placed parallel or nearly so, to a merging maneuver area to encourage merging at low relative speed. The major difference in opinion concerning acceleration design stems from lack of information on driver performance. Field observations have indicated that drivers desire to follow the direct path even though extra width or tapered section is provided. The length of acceleration lanes are determined by two factors: (1) Time required for drivers to accelerate to the speed of the preferential flow from the speed of entry into the acceleration lane and (2) Maneuvering time required as a supplement to the sight distance which is provided in advance of the acceleration lane. Taper distances are based upon a lateral transition time of about 1/3 sec/ft of displacement. Figure 1: Different forms of Acceleration lanes Figure 2: details of length of acceleration lane Deceleration Lanes Deceleration lanes are defined as extra pavement of constant or variable width, placed parallel or nearly so, to a diverging maneuver area to encourage diverging at low relative speed. The lengths of deceleration lanes are based on the difference in the speed of traffic of the combined flow (in advance of the collision area) and the speed at which drivers negotiate the critical diverging channel curve, as well as the deceleration practices of drivers. These deceleration lane lengths are based on the assumed performance of passenger vehicles only. Extra allowance must be made for grades and for trucks with different deceleration characteristics. In the figure below, Form A design is more economical when large speed differentials are to be overcome. Form B could be advantageous by contrasting pavement colors and Form C design is more convenient for drivers when small speed differentials are to be eliminated. Figure 3: Different forms of Deceleration lanes Figure 4: details of length of deceleration lane