According to NJDOT’s Smart Transportation Guidebook, many different factors come into play when choosing a desired operating speed for a roadway. From the discussion of speed:
“Desired operating speed is best explained by its relationship to three other concepts of speed: operating speed, posted speed, and design speed.
Operating speed is the speed at which a typical vehicle operates, commonly measured as the 85th percentile speed of all vehicles.
Posted speed is the legal speed limit on a roadway. It is often set without any means of self enforcement, and drivers tend to travel at what they perceive as a safe speed regardless of the posted speed. Fewer than a third of drivers go the speed limit on urban and suburban arterials.
Design speed (as defined in the AASHTO Green Book) is the speed used to determine various geometric design features, including horizontal curvature, gradient, superelevation, stopping sight distance, and, for rural highways only, lane width.
Historically, New Jersey has required the design speed to be 5 mph above posted speed for existing roadways, and 10 mph for new roads.
The greatest drawback to the existing design speed approach is that drivers usually drive as fast as they believe the road can safely accommodate.
Existing policy may thus encourage operating speeds higher than the posted speed limit and/or selected design speed in an area.
In the interest of highway safety, it is desirable to have a stronger relationship between the posted speed limit, design speed, and operating speed. Therefore, this guidebook recommends that the desired operating speed for most roadway types be the same as the design speed, and also the same as the posted speed.”
According to the Rt 571 Concept Design reviewed by county engineers in December 2009, the Main Street Princeton Junction design speed is 45mph, posted speed is 40mph, keeping the same values as exist currently.
Vehicle speed affects pedestrians’ safety in a number of different ways.
Likelihood of Collision
?Faster speeds increase the likelihood of a pedestrian being hit,? according to Federal Highway Administration’s Pedestrian and Bicycle Information Center.
Severity of Collision
If a pedestrian is hit by a vehicle traveling at 40mph, he/she has a 15% chance of survival, but if the vehicle is going 30mph, chance of survival increases to 55%, according to the Federal Highway Administration’s Pedestrian Facilities Users Guide: Providing Safety and Mobility (2002).
Crosswalk Compliance
Last, motorist compliance with yielding to pedestrians in crosswalks is significantly improved by reducing vehicle speed to below 35mph, according to the federal report Improving Pedestrian Safety at Unsignalized Crossings.
The WWBPA has requested the township to work with the county to follow the new NJDOT approach (called Context Sensitive Design) to choosing a desired operating speed, to support our emerging Main Street. NJDOT guidelines for a Community Arterial in a Suburban Center or Town Center context call for a desired operating speed of 25 – 30mph. Please support the WWBPA by contacting our officials, or write us an email at [email protected].
Tags: AASHTO, Complete Streets, Downtown Princeton Junction, Livable Communities, Route 571, Smart Transportation, speed limits, traffic, walking
Mostly on target. I have a few issues with a few things you said, however.
As you basically stated, if speed limits are to be lowered, the roadway itself has to support the speed limit as the “maximum safe speed” under ideal conditions. Which means that it should generally be set higher than the speed of traffic in open conditions by a bit, but not by enough as to give people the wrong message. Lowering the limit alone changes little as you have also stated.
As for the chances of a pedestrian being hit rising with speed, this isn’t true. Speed itself doesn’t result in a higher chance of someone being hit. Speed combined with inattention or poor driving does result in a higher chance of a pedestrian being hit. Driving inappropriately fast for the conditions (which does vary with the vehicle and driver) also raises the chance of a pedestrian being hit. I’d say that excessive speed raises the chance of someone being hit, I guess.
As for the chance of survival, this is also misleading. To hit someone at a speed, one has to take into account the slowdown from the original speed the driver was traveling at. In the unlikely event that an attentive driver not only fails to avoid a bad situation but fails to avoid the pedestrian (usually inattentive drivers do this), they will usually slow down substantially before they hit the person. Someone would likely have to be going 65 or so to hit someone going 40, or 50-55 to hit someone going 30 if they had poor reaction time and failed to recognize the hazard before hand. So the speed of collisions or the chance of collisions has little to do with the speed limit, and more to do with the driver and to a lesser extent for the vehicle. Unfortunately, often mainstream studies on this ignore things like this and use outdated braking standards as well as others. Again, I agree with the majority of your post. I don’t necessarily see the need for a design speed of 25-30 mph, as this may actually cause increased congestion because the road would need to be heavily modified and narrowed to support an ‘absolute maximum’ of 35 or something. I do however see the need for better crosswalks and such. More to come on this; I have some things to do.