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.
What if the major road has more than two lanes in each direction?
Add roughly 0.5 additional seconds to the base time gap for each lane beyond the first that the departing vehicle must cross — this calculator's lane-count adjustment applies that directly. Wide arterials with medians can require noticeably more sight distance.
Does this apply to a driveway or private access intersecting a public road?
The same AASHTO Case B methodology is commonly applied to driveways by many DOTs and access-management guidelines, though some agencies substitute a shorter, driveway-specific time gap. Confirm which is required by the reviewing agency.
Why does grade only matter above 3%?
AASHTO's base time gaps already assume a roughly level minor-road approach; below about 3% the effect on acceleration is small enough to be within that built-in margin. Above 3%, the extra time needed to accelerate away from the stop bar becomes significant enough to require an explicit adjustment.
What's the difference between Case B and Case F (left turn from the major road)?
Case B evaluates sight distance for a vehicle departing the minor road; Case F evaluates sight distance for a vehicle on the major road waiting to turn left across opposing major-road traffic. They use different geometry and different vehicle assumptions.
My calculated sight distance can't physically be achieved due to a building or embankment — what are the options?
Options include relocating the intersection, adding sight-distance easements, regrading or clearing the obstruction, restricting permitted movements (e.g., right-in/right-out only), or installing a signal or all-way stop to remove the gap-acceptance requirement.