E 551 General Design Factors

With varying degrees of influence, four principal factors determine the characteristics of an intersection. These are:

1. Human factors.
2. Traffic factors.
3. Physical factors.
4. Economic factors.

Some of the elements that make up these factors have effects on the design of the intersection that are self-evident and require little or no explanation. Others require a more detailed study than is warranted in this Part of the Manual.  Reference should be made to the traffic engineering texts for more detailed information. Since the street designer may be directly involved with the accumulation or use of some of these data, a short commentary is added at the end of the list of elements comprising each factor.

E 551.1 Human Factors

While drivers do not all act the same under all conditions, all drivers, to a greater or lesser extent:

1. Act according to habit.
2. Conform by following “natural” paths of movement.
3. Are susceptible to confusion from surprise.
4. Require adequate perception and reaction time.
5. Possess the ability to make correct decisions when laced with alternatives.

E 551.11 Compensating for Human Weaknesses

Corrective measures which tend to compensate for human weaknesses are:

1. Adequate sight distances.
2. Grade separations.
3. Smooth vehicular paths of travel.
4. Channelizations, or the design of roadway lanes, islands and medians to safely move vehicular traffic.
5. Paths with a minimum area of vehicular conflict.

E 551.2 Traffic Factors

The factors to be considered include:

1. Channelization — capacity of channels relative to traffic flow.
2. Turning movements to be permitted and accommodated.
3. Size and operating characteristics of vehicles.
4. Vehicular speeds.
5. Controlling traffic flow at points of intersection convergence and intersection divergence.
6. Transit operations.
7. Pedestrian movements.
8. Traffic (Accident)¹ Collision experience.

The Department of Transportation usually supplies most of the above information in the form of flow diagrams and traffic (accident)¹ collision data. These data are used in determining the relative importance of conflicting traffic movements; the geometric requirements of the intersection, such as left-turn pockets, medians, islands, and traffic controls; and cost-benefit ratios for establishing priority of construction, grade separations, etc.

E 551.21 Flow Diagrams

The average daily traffic (ADT) and design hourly volume (DHV) are indicated by the traffic flow diagram. It also indicates the time of day (a.m. or p.m.) of vehicles passing through or making turning movements within an intersection. The flow diagram states the year for which the indicated volume is anticipated, and the expansion factor used, or to be used, in arriving at the volume for the design year. Some of the other factors have been previously discussed or defined. See Sections E 310, Traffic Data, and E 311, Volume.

Major movements as determined from the flow diagram should be given preference. To accomplish this, the type or traffic control devices to be used would have to be known prior to intersection design.  Giving major movements preference may mean elimination of minor movements and may also affect channelization. Minor traffic movements may become a factor where there is a turning volume of 200 or more vehicles per hour (VPII) and where opposing through traffic exceeds 800 VPH during this same hour. A special design capacity study must then be made which would include traffic signalization and phasing control. When pedestrian traffic volumes are sufficient to affect the design, complete information on these present and future movements is necessary. All of the vehicular and pedestrian flow patterns must be considered when attempting to establish traffic control.

E 551.22 Traffic (Accident)¹ Collision Records

According to AASHO, “about 27 percent of all fatal accidents in urban areas involve collisions with other motor vehicles and more than one-half of these are angle accidents, indicating that the principal locations of accidents due to congestion in cities are at intersections at grade”. See Section E 020F(1d).

Many of these intersection (accident)¹ collision are a result of attempted left-turn movements. It is evident that all the types and causes of intersection (accident)¹ collision should be recorded and a determination of appropriate corrective measures included in the design study.

E 551.3 Physical Factors

In general, physical factors which control intersection design are topography, existing improvements, and physical requirements of the street or highway. These factors involve the following:

1. Total intersectional area.
2. Areas of possible conflict.
3. Intersectional angles.
4. Sight distances.
5. Speed-change areas.
6. Roadway grades, surface types, and cross sections.
7. Character and use of abutting property.
8. Need for traffic control decrees.
9. Number, size, and shapes of islands.
10. Need for and availability of lighting. 

Information regarding the topography and existing improvements is needed to help determine the amount of right of way, the grade separation feasibility, the extent of profiles and cross-sections required, and the optimum angle alignment of the intersections.

E 551.31 Intersection Topography

Information regarding topography is generally available in the form of land (ground) surveys, AL maps (field work shown on topographical maps), and aerial photographs. This information is provided by the Survey Division of the Bureau of Engineering at the designer’s request.

The proposed type of intersection determines the form and extent of the survey information required. Generally, where no additional right of way is needed, the normal intersection requires only a ground survey of the existing or proposed intersection and a profile and cross-section of the intersecting streets. For intersections requiring additional right of way, AL maps may be more advantageous. In making alignment and channelization studies, consideration should be given to the use of aerial photographs if it is anticipated that a ground survey will cost over $500.00.

It is suggested that where doubt exists, the designer should have a preliminary discussion with a survey supervisor of the Survey Division to determine the best method of survey for a particular project.

E 551.4 Economic Factors

Intersection design is often controlled by important economic factors. These are:

1. The cost of the improvement.
2. The economic effect on abutting businesses where channelization restricts or prohibits certain vehicular movements within the intersection area.
3. Benefits to the street or highway users.
4. Money available.

Cost estimates of right of way parcels which may be affected by alternate designs are essential. Right of way cost data are normally obtained from (Bureau of Right of Way and Land)¹ NavigateLA.

Footnote

¹The text in parenthesis is from the legacy Street Design Manual text and has been superseded by the text that follows.