Grade Separation

Intersections handling a high volume of traffic and pedestrians (and possibly railroads) limit the capacity of the approaching roads. Grade separating these conflict points using overpasses or underpasses allows traffic to flow freely. This in turn makes conditions safer for vehicles, pedestrians, and trains. Three roadway improvement objectives are achieved using grade-separated intersections:

• Increased capacity and constant flow.
• Increased safety.
• Reduced vehicle–train conflict and delay.

Overpasses increase the capacity of a roadway by allowing constant flow in all directions. The intersection approaches bypass each other, avoiding direct contact. Fewer signals are needed to direct traffic, thus decreasing the backup caused by signals.

Executive Summary

Target Market

Major Intersections with High Traffic Volumes
Intersections are a large cause of congestion on major streets. The signal time given to each direction decreases a road’s capacity, increasing the possibility of congestion. The stop-and-go condition reduces safety and increases travel time for all drivers. Elevating one of the streets reduces the conflict caused by intersecting roadways. The reduced conflict increases the road capacity.

Railroad Crossings
Railroad crossings pose a threat to the safety of street and rail traffic. Same-level crossings, especially near intersections, create many conflict points for cars, pedestrians, and trains. These crossings force both cars and trains to slow down, increasing travel time and congestion for cars and decreasing overall efficiency for the rail network.

How Will This Help?

Grade-separated intersections increase capacity by eliminating the delay caused by a previous intersection or railroad. Traffic moves freely, and signals may only be necessary for accessing the exit and entrance ramps of the interchange.

Elevating one portion of a street or rail crossing improves safety by eliminating vehicle, pedestrian, and train conflicts. The chance of collisions is decreased because:

• Crossing traffic is minimized.
• Trains are separated from the roadway.
• Pedestrians cross traffic less frequently.

The American Association of State Highway and Transportation Officials (AASHTO) Highway Safety Manual reports the benefits of changing to a grade-separated interchange. Converting a four-way stop to a grade separated intersection reduces injury crashes by 57 percent. Converting a signalized intersection reduces injury crashes by 28 percent.

AASHTO’s A Policy on Geometric Design of Highways and Streets³ describes six conditions, or warrants, that should be considered when determining if a grade separation or interchange is needed at a site:

1. Design designation—if the goal is to develop a highway with full access control between selected locations.
2. Reduction of bottlenecks or spot congestion—if insufficient capacity at the intersection of heavily traveled routes results in intolerable congestion on one or all approaches.
3. Reduction of crash frequency and severity—if some intersections have a very high rate of serious crashes, and inexpensive methods of reducing crashes are likely to be ineffective or impractical.
4. Site topography—if grade-separation designs are the only type of intersection that can be constructed economically.
5. Road-user benefits—if the road-user costs from delays at congested intersections are very high. Road-user costs include:
   • Fuel and oil usage.
   • Wear on tires.
   • Delay to motorists.
   • Crashes that result from speed changes, stops, and waiting.
6. Traffic volume warrant—if a traffic volume warrant may be the most tangible interchange warrant. Although a specific traffic volume at an intersection cannot be used as the only measure to determine the need for an interchange, it is an important variable. This is especially the case when combined with the traffic pattern and the effect of traffic behavior.

Not all warrants for grade separations are included in the warrants for interchanges. Additional warrants for grade separations include ones that will:

• Serve local roads or streets that cannot end outside the right-of-way limits of freeways.
• Provide access to areas not served by frontage roads or other means of access.
• Eliminate a railroad–highway grade crossing.
• Serve unusual concentrations of pedestrian traffic (e.g., a city park developed on both sides of a major roadway).
• Serve bikeways and routine pedestrian crossings.
• Provide access to mass transit stations within the borders of a major roadway.
• Provide free-flow operation of certain ramp designs and serve as part of an interchange.

Funding these relatively large projects remains the single biggest implementation issue. Still, the benefits are often well worth the cost. Finding locations with available right of way and public approval can be difficult, especially in heavily developed areas. Sufficient right of way should be gained when the intersection is first built. Buying additional land later greatly increases cost.

Making space for traffic during construction is also an important consideration. Rarely is an intersection completely closed when a grade separation is constructed. The needs of drivers who use the intersection must be planned for when scheduling the various phases of the project to allow traffic to proceed through the area as safely and efficiently as possible.

Who Is Responsible?

City and state officials are responsible for designing, constructing, and maintaining the overpasses and other features of the grade-separated intersection, depending on road jurisdiction. Cities should follow state codes for construction.

Cost

The project cost depends on the type of interchange being built. Building a two-level interchange or overpass typically costs $10 million to $30 million. This is low when compared to the cost of more than $100 million to build a larger three-level interchange. Additional right-of-way requirements may increase the cost greatly.

The entire FM 306 project is estimated to cost almost $40 million. This includes adding two railroad grade separations and widening 2 miles of the highway from two lanes to four. The two railroad grade separations have estimated costs of about $10 million and $15 million.¹

Project Time Frame

The time frame of a grade-separated intersection project depends on the size of the constructed overpasses. A project that includes building only a two-level roadway interchange or a railroad overpass will have a shorter time frame than a three-level interchange. The timeline for a grade-separation project typically takes place over multiple years to allow for:

• Designing the grade separation.
• Acquiring the right of way.
• Receiving any needed environmental clearance.
• Securing necessary funding.
• Deploying the construction phasing.

As mentioned previously, the FM 306 project is tentatively scheduled to let in August 2012,¹ and the duration of the construction period has not been specified. The concept of the project has been discussed since 2008, and design work took place in 2010.¹

Data Needs

Designing and constructing an adequate grade-separated intersection requires a large amount of information on the current interchange. Useful data include:

• Traffic counts for all directions approaching the intersection.
• Current information on signal timing and preemption (to be sure no other methods of re-signalization are possible).
• Peak-hour volumes for each intersection approach.
• Intersection geometry.
• Available right of way.
• Proximity to other intersections.
• Turning movements.
• Pedestrian volumes.

If the grade separation is due to a railroad crossing, additional useful data include:

• The average delay to traffic and trains.
• The frequency of trains.
• The number of car–train collisions.