Implementation Process: Appendix BB

Example Application of the Unsignalized Intersection Guide
(Source: Mississippi DOT)

The following material was part of a report of the Mississippi DOT team that participated in the demonstration of the first set of AASHTO Guides that were produced. It demonstrates something akin to a qualitative safety-audit approach to the use of the guides.

Investigation of Safety Conditions and Identification of Countermeasures

After eliminating misidentified crashes, a final list of high crash rate intersections was produced. To get a variety of intersection types in the study, the sites selected weren’t all on the top of the list. Six sites were picked to be studied. They included three urban intersections and three rural intersections. Site inspections were held in April 2002.

Four members of the project team inspected the sites, and following are recaps of the investigation of a typical rural and a typical urban intersection.

TYPICAL RURAL INTERSECTION – The first site was a rural intersection of a 4-lane US highway and a 2-lane state highway located in a rural community. The posted speed limit of the 4 lane route through the community is 55 mph. Just outside the community, the speed limit increase to 65 mph. The posted speed limit of the state highway approaching the intersection is 35 mph. The AADT of the main highway is 10,000 and the AADT of the side road is 4100. The US route was constructed in 1980 with the following design features:

Design Speed:	70 mph
Lanes:	Four 12’ lanes (2 in each direction)
Median width:	64 ft.
Access:	Partial Control (access at median crossovers 880’ apart)

Sight flares were provided at the time of the original construction. These provide stopping-sight triangles for approach speeds of 65 mph on the US route and 45 mph on the intersecting state highway. The terrain is level. Advanced intersection warning signs are in place on the main highway including a 45 mph advisory speed sign. Overhead flashing amber lights are in place for the main highway, and flashing red lights are shown to the minor roadway.

The crossovers are the standard type used extensively throughout the state. They consist of a fifty foot wide opening through the median as view from the side road. Bullet nosed ends are used. Left turn lanes with 225’ of storage are provided for left turn movements from the main highway onto the side road. The standard crossover pavement markings used on highways with 64 ft. medians does not have center yellow stripes for cross traffic. This particular intersection has had additional pavement markings added to provide delineation through the crossing area. This eliminates some instances of multiple vehicles "doubling up" in the cross over waiting to cross the additional lanes or enter the opposite lanes on the 4 lane facility.

There was an average of 5 crashes a year at this site. The crash data indicate the majority of the crashes were right angle type impacts. None of the crashes involved fatalities, however most had multiple injuries. Any of the right angle type crashes have the potential for fatalities due to the high speeds. Speed studies were not done as a part of this project, however while the project team was inspecting the site, a number of vehicles exceeded the post speed limit of 55 mph, and the vast majority exceeded the 45 mph advisory speed.

Rumble strips are not in place on the minor route at the approach to the intersection. No roadway lighting exists.

The various countermeasure strategies provided in the guide book were studied. Following summarizes the different objectives presented in the guidebook and the applicability of the strategies suggested.

Improved Access Management – The access is already controlled to the point that there are no existing driveways in close proximity to the intersection.

Reduce Frequency and Severity of Conflicts – Left turn lanes are already provided, and the storage lengths appear adequate. Right turn tapers are provided to help get right turners out of the travel lane. Adding right turn lanes might reduce the number of rear end collisions, however the majority of the crashes at the site are right angle impacts, rather than rear end crashes. The intersection is already channelized, so the turn maneuvers are somewhat restricted. Closing the intersection is not a feasible solution. The skew of the intersection is slight, and sight distance is not a problem, so realigning the cross road should not be considered. Pedestrians and Bicycles are not a problem.

Improve Sight Distance – Sight distance at the intersection is adequate.

Assist Drivers in Judging Gap Sizes – Due to the presence of speed violators and the fact that gap acceptance varies greatly from individual to individual, the concept of providing acceptable gap devices to improve safety was not considered as a strategy.

Improve Driver Awareness of Intersection – Signing and delineation appear to be adequate. Advanced intersection warning signs are in place on the main highway, along with 45 mph advisory speed signs. The minor road has advance "STOP AHEAD" signs. Each approach on the minor road has two stop signs, one on each side of the road. The addition of rumble strips on the minor roads would enhance driver awareness of the intersection. Additionally, roadway lighting of the intersection would increase driver awareness.

Choose Appropriate Traffic Control – Flashing beacons are present (amber for main route, red for side road). Traffic volumes are not sufficiently high to warrant signals or 4-way stops at this location. Converting the intersection to a roundabout would not be feasible at this location. It has been the MDOT’s experience that converting existing at grade intersections into roundabouts has not produced effective results.

Improve Driver Compliance and Reduce Operating Speeds – Automated enforcement of stop-sign violations was not considered due to legal questions. Providing targeted speed enforcement would help reduce the number of speed limit violations. Altering the roadway geometry to produce traffic calming is not appropriate in a rural area such as this. Public information campaigns for this intersection would be of have limited effects since the local community has no media outlets.

TYPICAL URBAN INTERSECTION – One of the urban sites studied by the project team is the intersection of a state highway and an access road to a entertainment/retail area in a suburban area. The intersection is located within 500 feet of an interstate ramp terminal. The intersection was constructed within the past 5 years. There was an average of 17 crashes a year for the years we studied. The types of crashes varied from sideswipes, rear end collisions and right angle impacts. No fatalities occurred in the years that were studied. The side road provides access to several restaurants, a movie theatre, a motel, and several other retail businesses. The posted speed limit on the highway is 45 mph and there is no posted speed on the side road. The AADT of the highway is 24,000 and the AADT of the side road is approximately 5000. There are several driveways located in close proximity of the intersection both on the side road and the highway.

The various countermeasure strategies were studied. Following summarizes the different objectives presented in the guidebook and the applicability of the strategies suggested.

Improved Access Management – Closing driveways or restricting turn movements from driveways would greatly improve access management, however the cost associated in closing or limiting access would be tremendous.

Reduce Frequency and Severity of Conflicts - Turn lanes are already provided at the intersection, and the storage lengths are adequate. Closing the intersection is not feasible due to the capital improvements present. The intersection is not skewed, so realignment is not needed.

Improve Sight Distance – Intersection sight distance is adequate.

Assist Drivers in Judging Gap Sizes – Since gap acceptance varies greatly the concept of providing acceptable gap devices was not considered as a feasible strategy.

Improve Driver Awareness of Intersection – Signing and delineation appear adequate. Awareness of the intersection doesn’t appear to be a problem. Rumble strips on the side road aren’t appropriate at this site. Roadway lighting is already provided.

Choose Appropriate Traffic Control – After all the strategies were investigated, it was decided that the best solution for the intersection is a traffic control signal. Adding a traffic signal isn’t always the best alternative. However, it appears that at this site it is the only feasible improvement.

Improve Driver Compliance and Reduce Operating Speeds – Speeding is not a problem at this site due to the congestion. Due to the commercial development in the area, speeds are low.