Advantages Roads with grade separation generally allow traffic to move freely, with fewer interruptions, and at higher overall speeds; this is why
speed limits are typically higher for grade-separated roads and grade separation is typically a prerequisite for the implementation of meaningfully
high-speed rail. In addition, reducing the complexity of traffic movements can reduce the risk of
accidents and further, reduce or preclude entirely the threat of
vehicular homicide and fatal cyclist-vehicle collisions that becomes statistically inevitable with a large enough population of pedestrians or cyclists crossing even a modestly trafficked
thoroughfare with reasonable posted speed limits. In the literal sense, only grade separation and the restriction of vehicle access to pedestrian spaces can actually and effectively reduce the probability of these deaths occurring regularly in any particular area to zero. While much less common and generally easier to prevent than automotive and truck collisions with cyclists and pedestrians, vehicle-train, cyclist-train and pedestrian-train collisions are almost exclusively fatal, particularly when involving
heavy or
freight rail, and avoidable only on the end of the collision's victim in the absence of grade separation in most cases. Regardless of the competency and alertness of a
train driver, there is nothing that the operator of a locomotive traveling at-speed can do to stop a train completely before reaching the most distant point on the tracks ahead of the driver that they were able to see at the point they first knew to apply the brake. This is considerably less true in relation to
light rail and
trams, which frequently operate in mixed traffic and as such are comparably lightweight and responsive to braking, able to come to a halt at roughly the same rates as would a bus or lorry (truck), and usually stop in less time than a loaded
semi-truck. While trains overall are relatively predictable and pass far less frequently than automotive traffic, these collisions still occur with some regularity, particularly at grade crossings. As such, grade-separated crossings for railroads are both less challenging and expensive to implement, and similarly result in improved safety for all parties, at least when the comparably low rate of train collisions compared to road deaths is not taken into account.
Disadvantages With roadway junctions in particular, grade-separated interchanges are typically space-intensive, complicated, and costly, due to the need for large physical structures such as tunnels, ramps, and bridges. Their height can be obtrusive, and this, combined with the large traffic volumes that grade-separated roads attract, tend to make them unpopular to nearby landowners and residents. For these reasons, proposals for new grade-separated roads can receive significant public opposition. Rail-over-rail grade separations, though, take up much less space than standard road or highway grade separations. In part, this is because shoulders are not required for railroad operations, even at high speeds, and there are generally far fewer branches and side road connections to accommodate because a partial grade separation will yield more improvement than it would for a similar road project, on which the overall traffic flow is determined by its most congested sections, as a result of well documented phenomenon such as
traffic waves. However, highway, mixed and even railroad-only grade separation projects, especially when 'retrofitting' an active transit corridor built without traffic conflict mitigations to save on construction costs, nonetheless usually necessitates considerable engineering expertise and effort, and can be very expensive and time-consuming to construct, especially when multiple environmental and existing-traffic related impacts must be studied, determined and adequately mitigated, as is required by law for projects of this nature in most jurisdictions. Grade-separated pedestrian and cycling routes often have a comparably modest footprint since they do not typically intersect with high intensity transit corridors (highways especially) that they would cross, without the safety provided by a grade-separated crossing. However, grade-separated pedestrian crossings with steps introduce accessibility problems and can potentially conflict with the
Americans with Disabilities Act in the United States. Some crossings have
lifts, but these measures can be time-consuming and inconvenient to use, and many of these
footbridges and pedestrian underpasses lie unused, abandoned and fenced off. Grade-separated roads that permit for higher speed limits can actually reduce safety due to 'weaving' (see below), the increased probability of collisions corresponding with
induced demand as well as the demonstrably false sense of safety caused by the monotony of driving long distances at high speeds with little or none of the stimulation and activity provided at-grade by stop lights, pedestrian crossings, more frequent turns and intersections. ==Roads==