Low-Cost Safety Enhancements for Stop-Controlled and Signalized Intersections

2. Stop-Controlled Intersections

Signing and Pavement Marking Enhancements

Crash Problem

The major crash type at stop-controlled intersections is a right angle crash involving a vehicle entering the intersection from the stop approach and a vehicle on the through approach. In these crashes, most of the vehicles on the stop approach stop or at least slow down to under 10 mph before pulling out. However, many drivers involved in these crashes make poor decisions regarding the available safe gaps between vehicles on the through road. Inattentive or distracted drivers, speeding, and physical limitations of the intersection contribute to future crash potential.

Countermeasures

The set of low-cost countermeasures for stop-controlled intersections is designed to increase drivers' alertness to the presence of the intersection and reduce potential conflicts with other entering vehicles. These countermeasures are primarily intended for deployment at stop-controlled intersections with either single through lanes or multiple undivided through lanes. Countermeasures have been classified as basic or supplemental. Basic countermeasures are those that are usually very low in unit cost and effective in terms of reducing future crash potential and should be considered at all intersections having crashes above a defined crash threshold. Supplemental countermeasures are targeted to intersections with crash levels considerably above the crash threshold or the intersection has specific types of crashes that the countermeasure can address.

Basic Countermeasures

The basic set of countermeasures should be considered as a package of minor improvements consisting of all of the following:

Low-Cost Countermeasures for the Through Approach

• Doubled up (left and right), oversize advance intersection warning signs, with street name sign plaques.

Low-Cost Countermeasures for the Stop Approach

• Doubled up (left and right), oversize advance "Stop Ahead" intersection warning signs.
• Doubled up (left and right), oversize STOP signs.
• Installation of a minimum 6 ft. wide raised splitter island on the stop approach (if no pavement widening is required).
• Properly placed stop bar.
• Removal of any foliage or parking that limits sight distance.
• Double arrow warning sign at stem of T-intersections.

Supplemental Countermeasures—Intersections with Higher Crash Frequencies

In addition to the basic package of countermeasures, additional individual countermeasures can be considered based upon higher frequencies of crashes beyond the crash threshold for basic countermeasures or at intersections that have crash types that the countermeasure can address.

• Installation of a minimum 6 ft. wide raised splitter island on stop approach which requires pavement widening. (See FHWA-HRT-08-063 for further design and performance information.).
• Either a) flashing solar-powered LED beacons on advance intersection warning signs and STOP signs or b) flashing overhead intersection beacons.
• Dynamic warning sign to advise through traffic that a stopped vehicle is present and may enter the intersection.
• Transverse rumble strips across the stop approach lanes in rural areas where noise is not a concern and running STOP signs is a problem. (Use "Stop Ahead" pavement markings if noise is a concern.).
• Dynamic warning sign to advise high-speed approach traffic that a stopped condition is ahead; use this countermeasure when vehicles running the STOP sign is a problem.
• Extension of the through edge line using short skip pattern may assist drivers to stop at an optimum point; this countermeasures is used on intersections with very wide throats in which stopped drivers have difficulty stopping at the correct location.
• Reflective stripes on sign posts may be used on signs with degraded conspicuity due to sign clutter or competing background features to increase attention to the sign, particularly at night. Supplemental countermeasures should be considered in addition to the basic set of countermeasures and not in lieu of the basic countermeasures on those intersections with higher crash frequencies or those that possess certain physical characteristics that the countermeasure is designed to impact.

Figure 1: Examples of Basic Low-Cost Countermeasures for Stop-Controlled Intersections – Double Up Oversize Warning Signs, Double STOP Signs, Traffic Island on Stop Approach (if feasible), Street Name Signs, Stop Bars, and Double Warning Arrow at the Stem of T-Intersections

Countermeasure Crash Reduction Factors, Threshold Levels, Additional Implementation Factors, and Estimated Cost Ranges

It is assumed that the existing traffic control devices at intersections under consideration for improvement are MUTCD compliant and usually consist of a single standard size intersection warning sign for each through direction of travel and "Stop Ahead" and STOP signs on the stop approaches. Even with these signs a high frequency of crashes may exist at the intersection. Applying the complete set of sign and markings, defined previously under "Basic Countermeasures," to the intersection is estimated to reduce future crashes by 30 percent (crash reduction factor (CRF) of 40). This estimate was developed by an expert intersection safety panel using past effectiveness research findings in combination with engineering judgment.

Crash reduction factors for supplemental stop-controlled intersection countermeasures have been taken from the FHWA Toolbox of Countermeasures and Their Potential Effectiveness to Make Intersections Safer and other FHWA publications. The CRFs, typical crash thresholds for application, additional implementation factors, and estimated cost ranges for each of the countermeasures are provided in Table 1.

The entire set of basic signing and marking countermeasures should be the primary improvement considered at stop-controlled intersections with a high frequency of crashes. One exception is stop-controlled intersections on divided multi-lane highways in which J-turn treatments (see next section) are the preferred countermeasure.

Additional supplemental countermeasures beyond the basic sign and marking enhancements should be considered at those intersections which either have much higher levels of crashes beyond the basic sign and marking crash thresholds or have other intersection crash concerns identified in Table 1 that may be addressed by a given countermeasure.

The crash threshold levels in Table 1 may be adjusted to reflect the following: entering traffic volumes (low volume intersections may have a lower threshold); and/or differences in severity rates (e.g., fatalities per 100 crashes) between urban and rural stop-controlled intersections. Refer to the Systematic Approach section of this document for further information on adjusting crash thresholds.

J-Turn Treatments at Stop-Controlled Intersections on Multi-Lane Divided Arterial Highways

Crash Problem

The major crash type at stop-controlled divided arterial highway intersections is a crash involving a vehicle entering the intersection from the stop approach and a vehicle travelling on the through approach, usually on the far side of the intersection from the right. The crash typically occurs after the vehicle from the stop approach has entered the divided median portion of the intersection and is attempting either to cross or turn left onto the far side of the arterial. While sign and marking countermeasures may impact this problem, they are not considered as effective as eliminating the through and left-turn movement from the stop approach and replacing them with a right-turn only J-treatment using a median break downstream.

Countermeasures

The low-cost countermeasure for multi-lane divided arterial intersections is to eliminate left-turn and through movements from the stop approach, forcing all vehicles to turn right. This is accomplished by adding minor channelization to the stop approach and in the median and supplementing the channelization with appropriate signs and markings. Drivers desiring to turn left or proceed through the intersection can make a U-turn a short distance downstream and continue on with their intended movements. Further information on J-turn designs and performance may be obtained from National Cooperative Highway Research Program (NCHRP) Project 15-30. This treatment is also referred to as either Superstreet or a Restricted Crossing U-turn intersection.

Figure 2: Turn Restrictions at Multi-Lane Highways

Countermeasure Crash Reduction Factors, Threshold Levels, Additional Implementation Factors, and Estimated Cost Ranges

Crash reduction factors for left-turn restrictions from stop approaches to divided highways have been taken from NCHRP Project 15-30, which included an evaluation of J-turn projects in North Carolina. Those evaluations concluded that there was a 100 percent decrease in cross-path crashes, a range of 72 to 84 percent reduction of frontal impact crashes, and an overall intersection reduction of total crashes of 43 to 53 percent.