15.5 Safety
At-grade intersections have the potential for crashes resulting from vehicle-vehicle and vehicle-pedestrian conflicts, in part due to conflicting crossing and turning movements that happen in a limited area. Grade separating these conflict points, using overpasses or underpasses, allow traffic to flow freely, in turn, making conditions safer for vehicles and pedestrians as well as trains.
Three roadway improvement objectives are achieved using grade-separated intersections:
- Increased capacity and constant flow;
- Increased safety; and
- 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 thereby avoiding direct contact. Fewer signals are needed to direct traffic, thus decreasing the backup caused by signals.
Grade separation increases roadway safety by reducing vehicle–vehicle and vehicle– pedestrian conflicts. The crossing traffic is removed from the intersection, which lowers the chance of collisions. Pedestrians are better protected from cars since they only have to cross one line of traffic. Also, more refuge points can be provided at multiple locations.
Removing intersections that serve both trains and vehicles substantially increases speeds and road capacity. Street traffic moves freely over or under the railroad tracks, eliminating wait times for a passing train. Most importantly, train–vehicle collisions are eliminated since the intersection no longer puts traffic in front of trains.
15.5.1 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.
15.5.2 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.
15.5.3 Benefits
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; and
- Pedestrian conflict points are reduced.
Refer to the
reports for 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.