Intersection Sight Triangle and Departure Distance

A driver waiting to pull out needs to see far enough along the through road to complete the manoeuvre before a vehicle arrives, and that distance is a time gap times the through speed. THE TIME GAP IS THE WHOLE DESIGN and it is a behavioural number rather than a physical one: it is how long a driver needs to start, accelerate, and clear the through lane without making an approaching driver slow. It grows with the number of lanes crossed, and it grows substantially for trucks -- which is why an intersection that works for cars can be inadequate for the farm, the quarry, or the plant it also serves. A passenger car turning left from a stop onto a two-lane road takes about seven and a half seconds; a single-unit truck takes about nine and a half, which at 45 mph is a hundred and thirty feet further in each direction. An intersection designed on the car gap and later serving trucks is deficient without anything having changed on the ground. The distance itself is then just speed times time, and it is longer than intuition suggests: seven and a half seconds at 45 mph is nearly five hundred feet each way. The area those legs enclose is large, and KEEPING IT CLEAR IS A MAINTENANCE OBLIGATION AS MUCH AS A DESIGN ONE. Landscaping grows, snow banks accumulate, and a permitted sign or a parked truck can defeat a properly designed intersection years after it was built -- which is why the triangle area is reported here, because that is the ground somebody has to mow. The vertical window is the detail people miss. Obstruction is judged roughly between three feet and eight feet above the road, because that is the band between a driver's eye and an approaching vehicle's roof: a low wall or a high canopy may sit inside the triangle without obstructing anything, while a hedge at four feet defeats it entirely. A sight distance calculation using AASHTO time gaps the reader supplies. NO GAP TABLE IS SHIPPED: the values depend on the manoeuvre -- left turn, right turn, or crossing -- on the design vehicle, on the number and width of lanes crossed, and on the approach grade, and the AASHTO Green Book tables govern. It does not evaluate approach sight distance, decision sight distance, or the criteria for signalized, yield-controlled and roundabout intersections, each of which has its own. It does not address horizontal and vertical roadway geometry that may obstruct sight independently of objects, and it does not measure the available distance, which is a field measurement taken at the actual driver eye height and object height. The legal status of an obstruction inside the triangle depends on right-of-way, easements, and local ordinance and is outside this entirely. The AASHTO Green Book, the agency's design standards, and the roadway engineer govern.

departure sight distance b = 1.47 x major road speed (mph) x total time gap (s), with the gap increased by an entered allowance for each additional lane crossed; the triangle is that leg along the major road against the decision point setback along the minor road, and its area is half their product.

The AASHTO departure sight distance relation and time gap values by name, with the AASHTO Green Book cited and the roughly 3 ft to 8 ft vertical obstruction window named. NO GAP TABLE IS SHIPPED: the gap depends on the manoeuvre, the design vehicle, the lanes crossed and the approach grade. The available distance is a field measurement at the actual driver eye height and object height. The AASHTO Green Book, the agency's design standards, and the roadway engineer govern.

One multiplication on a time gap the reader takes from the Green Book; no gap table is reproduced.

Estimate. AHJ and licensed professional govern.

Field names used by the API: major_speed_mph, time_gap_s, extra_lanes, added_gap_per_lane_s, setback_ft, available_left_ft, available_right_ft, total_time_gap_s, required_distance_ft, triangle_area_ft2, shortfall_left_ft, shortfall_right_ft, adequate_speed_mph

• The time gap is entered, not tabulated here it depends on the manoeuvre, design vehicle, lanes crossed and gradethe AASHTO Green Book
• Stop-controlled departure only signalized, yield-controlled and roundabout intersections each have their own criteriathe AASHTO Green Book
• The available distance is measured in the field at the actual driver eye height and object heightthe agency's design standards