National Boards > Traffic Control

Best configuration for service roads with slip ramps?

(1/3) > >>

webny99:
For freeways that have a service road (or "frontage road" or whatever other term you may call it), what is your preferred exit/entrance ramp configuration?

The two main configurations I'm looking at are exit first, where the weaving movement between exits occurs on the service road, and entrance first, where the weaving movement between exits occurs on the mainline.

The obvious advantages of "exit first" are less disruption to mainline traffic flow and better access to waypoints between exits, which in turn reduces traffic at intersections along the service road. The service road can also be used to bypass a section of the mainline in the event of an incident. The obvious drawbacks are overlapping exits which create an extra decision point for drivers, and the potential need for auxiliary lanes at each over/underpass when exits are closely spaced.

The obvious advantages of "entrance first" are minimal lanes at over/underpasses, less through traffic mixing with local access traffic, and less potential for driver confusion. The obvious drawbacks are that weaving could disrupt mainline traffic flow, and there's more traffic at the intersections along the service road due to traffic accessing waypoints, and in the event of mainline congestion.

Also worth noting that Ontario does something entirely different, with two 2-way service roads that are independent of the mainline and go around each interchange entirely.

Curious if there's other common configurations that I haven't considered here, and if anyone has thoughts on which configuration they prefer and why.

roadfro:
I much prefer when the weaving occurs on the frontage road and not the mainline. That weaving happens at (theoretically) slower speeds so it's a bit safer, and keeps the freeway flowing much more smoothly.

The key to this being a successful setup is making sure the exit ramp is located sufficiently upstream from the intended cross street, especially if it's a multi-lane frontage road, so that exiting vehicles turning right have sufficient distance to merge across the frontage traffic to the right lane.

J N Winkler:
I don't really have a general preference one way or the other.  It all depends on how well the various segments of roadway handle the traffic volumes they see, and also the role I am playing as driver.  For example, if I am familiar with an area that is prone to traffic congestion and looking to turn right off the frontage road into a business, I like the X-ramp configuration (freeway entrance before crossroad, exit after) since it gives me more room to maneuver across the frontage road.  But if I have no local knowledge (e.g., am visiting from out of town), X-ramps are less intuitive since they mean I have to exit long before I can see the crossroad or the business I am looking for, and the added length of frontage road travel can be irritating when volumes are light.

Dirt Roads:
The one thing about the "entrance first" arrangement is that this can function more like traditional off-ramps on a freeway (if arranged such that the "entrance" comes immediately after one overpass/underpass, and the "exit" comes immediately before the next overpass/underpass).  As such, the mainline signage works just like freeway signage and there is no obvious reason to sign the exits to acknowledge the service road.  Almost always, the "entrance last" arrangement results in a number of exits that are nowhere near the next overpass/underpass and the only logical name for the exit is "Service Road".  Which really doesn't help me figure out whether it makes sense to get off here or go to the next traditional exit where I can see the tall signs that are closest to that overpass/underpass.

webny99:
I agree with everything that's been said so far and would add that the density of direct access points along the service road is also a contributing factor. I'm used to very low density - in fact the entirety of NY 104's 2.5-mile service road in Webster has only two direct (non-interchange) access points, one eastbound at Barrett Drive, and one westbound at North Ponds Park. The NY 104 service road in Irondequoit has a similar low density of access points - I'm counting five total in a similar 2.5 mile segment.

Navigation

[0] Message Index

[#] Next page