Street Grade and Intersection Sight Distance on a Curve
A proposed 1,100-ft residential collector street climbs a 6% grade and ends in a T-intersection with an existing arterial. The vertical curve at the crest of the collector is a 300-ft sag/crest combination; the crest has a K-value of 90 for a 30-mph design speed. The intersection approach is on a 4% upgrade. A 6-ft sidewalk and a 5-ft landscape strip are proposed on both sides. The arterial has a posted 40-mph speed limit and carries 12,000 vpd. The developer's engineer shows the intersection meeting stopping sight distance but does not address intersection sight distance or the crest.
1. Evaluate whether stopping sight distance alone is sufficient at this intersection, and identify what else must be checked.
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Stopping sight distance (SSD) is necessary but not sufficient. At an intersection, the driver on the minor (collector) approach must be able to see and be seen by conflicting arterial traffic, which is intersection sight distance (ISD), a different and generally longer criterion. For a 40-mph arterial, the required ISD along the arterial is on the order of 445 ft (based on a 40-mph design speed and typical ISD criteria), measured from the driver's eye 10 ft back from the edge of the arterial travel lane. The engineer must confirm that the approach sight triangle is clear of obstructions - landscaping in the 5-ft strip, signs, and the crest of the collector - and that the arterial's horizontal and vertical geometry provides the required sight distance in both directions. The crest with K=90 at 30 mph should be checked against the SSD for the collector's design speed; a K of 90 may be marginal depending on the design speed and whether it is a crest or sag. The engineer must also check the intersection's vertical alignment so the approach is visible, and confirm the 4% upgrade does not reduce available sight distance.
2. The crest K-value is 90. What does that imply, and how should the engineer verify it is adequate?
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For a crest vertical curve, the minimum K-value is set by the required stopping sight distance for the design speed: K = L/A, and the SSD relates to K through the sight-distance equation. For a 30-mph design speed, the required SSD is about 200 ft (using standard AASHTO values), and the minimum crest K is roughly 37 for SSD with an assumed driver eye and object height. A K of 90 is therefore well above the minimum for 30 mph and should provide more than adequate SSD - but the engineer must confirm the design speed, the assumed eye and object heights, and whether the crest is on the collector approach to the intersection where ISD, not SSD, governs. The real check is whether the crest blocks the view of the intersection or of arterial traffic from the minor approach. The engineer should plot the sight line from the driver's eye at the stop location over the crest to the conflicting traffic, and verify the crest does not intrude into the required sight triangle. If it does, the crest must be flattened (higher K), the intersection relocated, or the approach regraded.
3. The 5-ft landscape strip is proposed along the intersection approach. What constraint does it create, and how should it be resolved?
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The landscape strip sits within the area where the intersection sight triangle must be kept clear. Landscaping - trees, shrubs, signs, and even tall groundcover - can obstruct the driver's view of conflicting arterial traffic and violate ISD. The strip must be designed so that plantings within the sight triangle stay below the driver's eye height (typically about 2.5 to 3 ft) at maturity, or the strip must be interrupted at the intersection corners. The resolution: delineate the sight triangle on the plans, specify low-growing vegetation within it, prohibit trees and tall shrubs, and coordinate sign placement so signs do not block sight lines. If the strip cannot be kept clear, it should be terminated short of the intersection and the sidewalk shifted to the back of curb. The engineer should show the sight triangle on the grading and landscaping plans and note the planting restriction so it is enforced at construction.