Speed limit theory, but not in OR

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    Idaho recently increased the speed limit on most Interstates to 80, and surprise, tickets are down.

    Utah just now has increased urban speed limits to 70, one of the highest urban speeds in the country. WA is 60, and CA is 65 in this category. OR remains at 55 in urban areas, and still 65 in rural areas. What a wide variety of speed limits among states that are bordering each other.

    The theory is produce speed limits that most are driving anyway, and enforce the hell out of them. Seems to be working. While Idaho is now 80, they don’t give much leeway above.

    But, back to Oregon. If the limit is lower than most of the region, doesn’t this increase the accident risk, as some will drive the limit and others will exceed, making accidents more probable? I am wondering if Oregon understands this. If they do then they need to enforce the granny speed limits otherwise, they are just opening up their highways for more accidents. Thoughts?


    80 MPH requires considerably more fuel to move “x” amount of weight than at 65, regardless of the design of the vehicle. The laws of physics will never change here.

    I don’t know about you but I don’t want to hand over a trump card to OPEC or some other mid-Eastern country, or God forbid, the Russians.


    Yes, but that curve of fuel consumption varies considerably.

    Coupla data points:

    My 78 Chevette: Similar to the Corolla below, but heavier and modified to use much taller gears than the stock model.

    Carb engine, with a fairly steep curve. At 45 on modest terrain, the car got very good MPG, 35 or so. Awesome for the time. Nothing made during that time in the US even came close, even other Chevettes. GM really missed it by not thinking through gear ratios on that model, IMHO Also IMHO, I’m probably the only guy who noticed.

    At 60 or so, I saw 25-30 MPG.

    Above that, the vehicle was kind of a pig.

    My old 89 Corolla: This is a light weight car, made before the significant safety factor bump required more car mass.

    Another Carb engine, but the consumption curve is a lot less hockey stick like in that vehicle.

    It is actually hard to measure the difference in MPG between 45 and 60 in that car. It’s consistently 35-40 MPG. The gearing in the car isn’t as tall as the Chevette was (lower RPM per tire turn), but it’s not short, like many small engine economy cars are. Toyota thought this one through.

    I have driven long distances, to Idaho actually, in that car, pegged at 70-85 the whole way up I-84. (yeah, I could have got a ticket, but I was seriously late for an event) I got 35 MPG, and I’ve gotten similar metrics on many longer drives in that car, to Southern Oregon, Tri-Cities WA, Seattle WA.

    That car has a modest computer that does tweak a few things the Chevette used only mechanical means to do.

    2000 Expedition. This thing is HEAVY. Typical SUV beast. It’s used for camping, travel with lots of people or stuff or both, and it goes off road / difficult weather regularly.

    It has a fairly sophisticated and adaptive computer control.

    MPG has a flatline at about 20 MPG. It doesn’t matter what speed or conditions, save for a serious amount of coastable down hill, that go above. Once in a while, I can get a nice peak of 22, but that’s after new service, filters, etc.. Doesn’t last long, and I’ve measured the crap out of this vehicle with computer data logging via the diagnostic service port I can’t seem to recall the name of right now.

    So, 35, 45, doesn’t matter. It’s gonna top out at 20 MPG, and it will probably do 18 under light load.

    That vehicle at 60 will get 15 – 17 MPG.

    At 80, it will get 14-16 MPG, and it’s like driving a freaking brick!

    Those are all light load, mixed terrain numbers from trips I do regularly. Most notably, I just did two round trips both 14-15 MPG, one foul weather, slow ride, 45 MPH average, the other quick, 65-70 MPH average, with very little overall difference in consumption. It’s literally a couple of MPG tops, with a modest load.

    That vehicle compensates for temp, load, driver style and demand, shift patterns, etc…

    (and I’ve been kind of a geek about this for a long time, which is why I have recall of these things, FYI)

    I now have a 2004 Kia, V6. It’s an interesting car, a lot lighter than the Expy, but with a larger engine, and I’m not sure how capable the computer actually is.

    Just starting to log some trips and collect real time data so I understand how to maximize the car.

    BTW, doing that logging in the Expy netted me about 5 MPG in town, load or not, just by understanding how to really drive and maximize that vehicle. And some of the techniques are not obvious. At least to me.

    What I can tell you is the curve on the Kia is somewhat aggressive. It will really suck down the fuel at 80, more so than the Expy will. And that does not entirely make sense. And it can’t touch the lighter, taller geared cars I’ve had in the past at modest speeds.

    Overall, the Kia is mediocre to OK on fuel consumption. They too didn’t choose tall enough gears for that V6, or they could have changed the spread up a little, or added an actual overdrive gear option. Price / performance on that car is great, given it’s retail cost, so I understand those choices, but really? How much more would the car be with just a shade more engineering? Annoying.

    So that’s what I know.

    I’m kind of shocked at the Expy performance given it’s absolutely horrid drag coefficient. The Kia has a much nicer one, but it doesn’t see the benefit of it like the Corolla clearly does.

    I suspect given a set of data from cars made today, or in the last 5 years would reveal those consumption curves to be a lot more modest than they were just 10 years ago, and very significantly more modest than all but a few odd cases or modified vehicles made before that.

    And most of that is just logged data. Did the drives, collected the data, once in a while, stuff it into excel to see what I see.


    BTW, that data tells me another thing.

    Hauling a well engineered SUV along at 75+ has it’s cost, and many of those are optimized in today’s vehicles.

    The big hog is city driving, stops, starts, basically delta-v changes. Inertia in that one is a significant factor in it’s economy and driving with that in mind can fill your tank for free in a mere few weeks of driving. It’s that significant.

    So, rather than hammer on 80 MPG consumption, we really should be asking ourselves how we can optimize the weight some more with better materials and put a battery assist on these things for the big win. From what I see, it could be a game changer, taking in town from single digits at worst case and very low teens in the average case to the high teens with the odd peak over 20MPG in more excellent cases.

    Doing that for heavy vehicles would get people seriously excited about electric power for cars. They will notice the difference big. If I were in a position to do it on the one I have, I seriously would.


    Really, we’ve got no realistic end game regarding OPEC.

    We either get electric cars done, viable and being adopted, or what tweaks we make to overall efficiencies aren’t going to do anything other than tweak the potentials a little this way or that.

    It sure won’t be anything material.

    Andy Brown

    Oregon has less law enforcement personnel per 100,000 population than any of those other states Paul mentioned and more miles of roads than Utah and Idaho. Also, remember in WA the State Patrol has only one responsibility and that is the state and federal highways whereas in Oregon, the State Police have a much larger area in miles of roads to deal with, especially since the conservatives in rural Oregon refuse to fund their local police and sheriffs.

    http://www.bjs.gov/content/pub/pdf/csllea08.pdf Fig. 4


    Rural vs Urban miles is significant. Check it out.


    Somehow I think raising speed limits in Oregon without more enforcement capability would not be a good idea. This is a political problem about funding to provide manpower. I don’t think the DOT in any state gives a hoot about your gas consumption.


    I don’t think they do either. That’s self-correcting from their point of view.

    The low enforcement argument is compelling.

    Frankly, it’s not bad now. The limits are low, but if one is prudent and reasonable about it all, there isn’t a high risk of a citation, but for some speed trap / high patrol, focus areas, and those are targeted on accident rates anyway.


    I am all for raising Oregon’s speed limit. 65 in rural areas, given the speed limit in all the surrounding states, is unacceptable IMO.

    That said, as Paul and others alluded to, enforce it to a T once you do. In the case of Oregon, which has a very thin corps of state troopers, they’d have to add to the budget and get more enforcement out there.

    They actually did just that on Airport Way, west of 205 leading up almost to the terminal. It was 35, and the Port decided to raise it to 45, with the caveat that it would be strictly enforced. It is a divided road, deemed very safe (no pedestrians, etc.), and most everyone was driving around 45 anyways. You see a lot more speed traps on Airport Way to go along with the higher speed limit, and it is working great.


    Raise the limit to 80 on the highways. It’s what people are driving anyway.

    Master of Disaster

    It’s all about the revenue. Raise speed limits and that’s one less source of funds and one less way to catch drunk drivers, drug smugglers, etc.

    As for gas prices, here’s the Master of Disaster’s Gas Price Prediction of Fail: as soon as the next congress takes office, gas prices will shoot to record highs while coverage of Ferguson demonstrators shifts to Keystone XL Pipeline support demonstrations, interest groups run campaigns for people to call their congressperson, and the House and Senate hit 2/3 supermajority to override any possible Presidential veto.

    If I’m wrong, hey, I did have the word “fail” in the name of the prediction! XD


    The US Interstates were designed for a minimum speed of 70mph (http://en.wikipedia.org/wiki/Interstate_Highway_standards). Of course, in certain areas design constraints merit a lower maximum speed.

    Many rural highways are NOT designed for much above 55mph. Curve radii and the lack of passing zones on most roads is what keeps most speed limits low, not politics.

    Andy Brown

    “Raise the limit to 80 on the highways. It’s what people are driving anyway.”

    People aren’t driving 80 in the urban corridors.

    In other areas people will then drive 90 and the curves are not engineered for that. It would be a disaster waiting to happen in more places than not.

    Raising the speed limit to 80 in urban areas would be a bigger disaster.

    Without more speed control (police), Oregon shouldn’t go anything more than 5 mph more, and not even doing that is not necessarily a bad decision. People go about ten over the limit (75) between urban areas. Exceeding that going north and your red meat for WSP. To exceed that in Oregon you have to south of Eugene. Doing 80 between Portland and Salem or Salem and Eugene is reckless because there is alway slower moving traffic. There are way too many cars on the road. Some of those other states in the initial post don’t have anywhere near as much traffic as OR and WA do, because of I-5 with the exception of the area around Salt Lake where staying on I-80 is somewhat convoluted. Seattle and Portland are a big mess unlike anything in Idaho and the rest of Utah. Sure, California has a higher limit but around the metros of Oakland/San Fran, LA, San Diego, no one is moving fast any time of the day so a higher limit is irrelevant.


    Actually, I’m in favor of getting rid of ALL speed limits and just abide by the basic rule.

    And speed limits should NEVER be dictated by fuel consumption. Speed limits should only be in place for safety.


    “Actually, I’m in favor of getting rid of ALL speed limits and just abide by the basic rule.”

    You know what? That’s actually the way that Oregon speed rules already work! If you look closely, almost all speed signs in Oregon say “Speed”, not “Speed Limit”. As long as you follow the Basic Rule (“Drive at a safe speed for the conditions”) you shouldn’t get a ticket. And if you get a speeding ticket, it will say “Violating the Basic Rule”.

    Ah, but here’s the catch: The posted “speed” is the speed rating for that stretch of road which considers intersections, driveways, crosswalks, line-of-sight blockages, road surface, etc. The posted speed is considered the maximum safe speed for that roadway. The ticketing officer can argue that you were violating the basic rule simply by exceeding the pre-determined maximum safe speed.


    Washington State now negotiating a higher limit on I-90 mainly in the eastern part of the state, which is mostly rural. The State will come to a vote to move from 70 to 75 on some stretches. This will increase the difference between WA and OR, but still fall below the rural speed limit of 80 in most of Idaho and Utah. I predict this will pass.

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