§ 15.01.1910. Intersection sight distance.

This section discusses recommended intersection sight distance for several intersection types and is based on AASHTO Greenbook (2004) recommendations. These recommendations are made for various intersection control scenarios as well as functional classifications of the intersecting roads. Additionally, sight distances to accommodate trucks have also been calculated. Scenarios discussed in the following sections include intersection sight distance for a full access stop-controlled intersection, a RIRO access, a yield-control intersection (four-leg and three-leg intersection), and an uncontrolled intersection. Sight distance requirements for a left turn from a major street onto a minor street are also given within the provided tables for illustrative purposes. All intersection sight distances should be calculated by a licensed traffic engineer and compared to the tables within this division.

For all of the scenarios, the following assumptions have been made:

(1)

Design speeds:

•

Access: Fifteen mph.

•

Local: Thirty mph.

•

Minor collector: Thirty-five mph.

•

Major collector: Forty mph.

•

Arterial: Fifty mph.

(2)

Intersections are flat (all slopes are assumed three percent or less).

(3)

Cross sections:

•

Local: Two lanes.

•

Minor collector: Two lanes.

•

Major collector: Three lanes.

•

Arterial: Five lanes.

(4)

All calculations are based on AASHTO Greenbook (2004) assumptions.

Because the grade of the major or minor street does affect sight distance, the values presented in the sections below should be adjusted when grades are more than three percent. These adjustments should be made on an individual case-case basis by consulting the AASHTO Greenbook. Additionally, alternative design speeds or cross sections will also require adjustments to these sight distances.

Once intersection sight distances have been determined for a given intersection, a "clear sight triangle" can be formed which should remain clear of anything that will obstruct the driver from seeing other vehicles within the intersection sight distance. The AASHTO Greenbook states that the minimum height of a driver's eye is approximately 3.5 feet. The maximum height (which corresponds to the driver of a combination or single unit truck) is approximately 7.6 feet. Therefore, objects should not be placed in the clear sight triangle between 3.5 and 7.6 feet in height. Objects such as buildings, fences, and parked cars are examples of objects that will block the intersection sight distance. Landscaping features such as shrubbery and decorative rocks are acceptable as long as they are less than 3.5 feet high. Trees are acceptable as long as the stump is located outside of the clear sight triangle and the lowest branches are at least 7.6 feet tall. These height standards apply to areas with flat grade. Intersections near slopes must be evaluated on a case-by-case basis.

A.

Stop-controlled intersections. Intersection site distances for stop-controlled intersections include full movement intersections where the minor street is stop-controlled, but the major street does not stop. RIRO intersection sight distance is also presented. Table 10 shows the sight distance required for full-movement intersections. Table 11 shows the sight distance required for RIRO intersections. In both cases, the sight triangle for the minor leg should be twenty feet back from the edge of traveled way of the major street. The value shown in the table is how far to the left and right for which the stopped vehicle should have a clear sight triangle. Figure 2 illustrates these distances and the sight triangle.

_____

Table 10. Intersection Sight Distance — Two-Way Stop-Controlled
Full-Movement Intersection (feet)

Design Vehicle	Major Street
	Local	Minor Collector	Major Collector	Arterial
Passenger car	335	390	470	625
Single unit truck	420	490	600	805
Combination truck	510	595	720	950
Notes:
1. Design speed: Thirty mph, two-lane cross section.
2. Design speed: Thirty-five mph, two-lane cross section.
3. Design speed: Forty mph, three-lane cross section.
4. Design speed: Fifty mph, five-lane cross section.

 

Table 11. Intersection Sight Distance — Stop-Controlled RIRO Access (feet)

Design Vehicle	Major Street
	Local	Minor Collector	Major Collector	Arterial
Passenger car	290	335	385	480
Single unit truck	375	440	500	625
Combination truck	465	545	620	775
Notes:
1. Design speed: Thirty mph, two-lane cross section.
2. Design speed: Thirty-five mph, two-lane cross section.
3. Design speed: Forty mph, three-lane cross section.
4. Design speed: Fifty mph, five-lane cross section.

 

Figure 2. Intersection sight triangles for stop-controlled minor approaches.

B.

Yield-controlled intersections. Intersection site distances for yield-controlled intersections include four-leg intersections where both minor street approaches are yield control as well as three-leg intersections where the minor street is yield-controlled. In both scenarios, the major street does not stop. Other combinations of yield approaches must be evaluated on a case-by-case basis. Table 12 shows the sight distance required at four-way intersections with minor-street yield control. Table 13 shows the sight distance required at T-intersections with one yield-controlled leg. For the T-intersection scenario (Table 13), the site distance on the minor street is measured eighty-two feet back from the traveled way of the minor street approach. Figure 3 and Figure 4 illustrate these distances and the sight triangles for four-way and T-intersections, respectively. Sight distance values were only calculated for passenger cars as it is assumed that roads with significant truck traffic will not be yield-controlled intersections. Also, it was assumed that the major streets do not have classifications higher than minor collector, as these types of high classification streets should at least have stop control on the minor street.

Table 12. Intersection Sight Distance — Four-Way Intersection
with Minor Street Yield Control (feet)

	Minor Street	Major Street
		Local	Minor Collector
Minor street leg	Access	75	75
	Local	160	160
Major street leg	Access	300	345
	Local	290	335
Notes:
1. Design speed: Thirty mph, two-lane cross section.
2. Design speed: Thirty-five mph, two-lane cross section.
3. Design speed: Fifteen mph, two-lane cross section.

 

Table 13. Intersection Sight Distance — Three-Way Intersection
with Minor Street Yield Control (feet)

	Minor Street	Major Street
		Local	Minor Collector
Minor street Leg	Access	82	82
	Local	82	82
Major street Leg	Access	355	415
	Local	355	415
Notes:
1. Design speed: Thirty mph, two-lane cross section.
2. Design speed: Thirty-five mph, two-lane cross section.
3. Design speed: Fifteen mph, two-lane cross section.

 

Figure 3. Intersection sight triangles for four-way intersections with yield control on the minor street approach.

Figure 4. Intersection sight triangles for three-way intersections with yield control on the minor street approach.

C.

Uncontrolled intersections. Uncontrolled intersection site distances are for those intersections where neither the minor nor the major street has any control. These streets are typically very low volume roads primarily in residential subdivisions. Table 14 shows the sight distance required at uncontrolled intersections. Only access roads and local streets were shown as any other higher classification road should have some intersection control. The distance shown in the table corresponds to the site leg of the site distance triangle for the respective type of road. Figure 5 illustrates the site distance triangle for this type of intersection. Sight distance values were only calculated for passenger cars as it is assumed that roads with significant truck traffic will not be yield-controlled intersections.

Table 14. Intersection Sight Distance — Uncontrolled Intersection (feet)

	Access	Local
Distance of approach leg	70	140
Notes:
1. Design speed: Fifteen mph, two-lane cross section.
2. Design speed: Thirty mph, two-lane cross section.

 

Figure 5. Intersection sight triangles for
uncontrolled intersections.

D.

Left turns from the major road. Intersection sight distance for left turns from the major road include any left turn movement where the driver must yield to oncoming traffic (through and right turning vehicles) before turning left onto the minor street. Table 15 shows the values for the sight distance triangles based as a function of design vehicle. Figure 6 illustrates the sight distance triangles for this type of movement.

Table 15. Intersection Sight Distance — Left Turns from Major Street (feet)

Design Vehicle	Major Street
	Local	Minor Collector	Major Collector	Arterial
Passenger car	245	285	325	445
Single unit truck	290	335	385	530
Combination truck	335	390	445	605
Notes:
1. Design speed: Thirty mph, two-lane cross section.
2. Design speed: Thirty-five mph, two-lane cross section.
3. Design speed: Forty mph, three-lane cross section.
4. Design speed: Fifty mph, five-lane cross section

 

Figure 6. Intersection sight triangle for a left turn from the major street onto the minor street.

(Ord. No. 2012-05-15, § 2, 5-8-2012)