AASHTO Intersection Sight Distance: Case B Explained
Intersection sight distance answers a different question than stopping sight distance. Instead of asking whether a driver can stop before hitting something, it asks whether a driver stopped at a minor-road approach can see far enough down the major road to complete a turn or crossing maneuver before a conflicting vehicle arrives. AASHTO's Case B covers the most common version of this problem: a minor road controlled by a stop sign, intersecting an uncontrolled major road.
Why a Time Gap Instead of a Fixed Distance
Rather than publishing a single sight-distance table, AASHTO builds Case B sight distance from a time gap — the number of seconds a departing vehicle needs to clear the conflict zone. Multiplying that time gap by 1.47 times the major-road design speed converts it into a distance in feet. This approach captures something a fixed table cannot: the time a vehicle needs to depart depends heavily on what kind of vehicle it is and what it's doing, so the sight distance requirement should scale with the maneuver, not just the road's speed.
Why Left Turns Need More Time Than Right Turns
A left turn from a stop requires accelerating across the near lanes of the major road and merging into the far lanes, a longer and slower maneuver than a right turn or a straight crossing movement. AASHTO reflects this with different base time gaps — typically 7.5 seconds for a passenger car turning left versus 6.5 seconds for a right turn or crossing movement. Trucks need still more time to accelerate to speed, so combination trucks add a full additional second to the base gap.
Grade and Lane-Count Adjustments
Two site conditions push the required time gap higher. An uphill minor-road approach steeper than 3% slows a vehicle's acceleration away from the stop bar, so AASHTO adds 0.2 seconds of gap for every percent of grade beyond that threshold. Crossing additional major-road lanes — common at wide arterials or where a raised median separates travel directions — also lengthens the exposure time, adding roughly half a second per lane beyond the first. Both adjustments exist because the physical act of clearing the intersection simply takes longer under those conditions, independent of the major road's design speed.
Where Case B Doesn't Apply
Case B assumes stop control on the minor road only. Signalized intersections, all-way stops, and roundabouts each have their own AASHTO sight-distance methodology, and yield-controlled approaches use a shorter time gap than stop-controlled ones. Applying Case B outside its intended condition will understate or overstate the actual sight distance needed, so confirming the intersection's traffic control is the first step before running this calculation.
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