Why are so many flat CA state highways signed at 55 MPH?

[Max Rockatansky]

This is something that has wracked my brain for years and I have never found a good answer.  I was thinking about this a couple weeks back on CA 138 in Antelope Valley and it came up in another thread today:

I'd say California except for where it caps out (I.e. 65 on 4 lane roads, cap of 70 on freeways). Any speed limit lower than the default must be justified by the 85th percentile (in general). While there are many exceptions (especially freeways), I'd say they're mostly good.

For other states, Arkansas and some of Texas is relatively good on the rural freeways, especially given their design. Not all parts of these states are good per se, but this is what I could come up with lol

Are you kidding?  There is so much 55 MPH two lane roads that could be bumped up to 65 MPH.  CA 43, 145 and 41 near me could all be switched tomorrow and it would just match the existing traffic flow. 

I am stupid, I completely forgot about some of the more-rural 2-lane roads.

Point changed to: California in more built-up / suburban areas. Rural areas still need a ton of work... maybe if the default was 10-15 higher?

Thanks for the catch

FWIW I've been trying to crack what exactly the code or standards Caltrans goes by for classifying two lane roads for 65 MPH speed limits.  North of Sacramento and in the desert regions they fairly common.  All the same some of them don't really make a lot of sense like the 65 MPH portion of CA 99 near Red Bluff.  I thought that it might have something to do with two lane expressway classification, but that seemingly isn't the case.

[Flint1979]

Caltrans can post a speed of up to 65 mph but they have to do an engineering study first.

[pderocco]

Caltrans can post a speed of up to 65 mph but they have to do an engineering study first.

What does an engineering study study? Do they have rules of thumb based on lots of statistics that predict accident rates from measurable road characteristics like curve radius, banking, or passing zone length? I would think it would be more about looking at the ratio between existing accident rates and AADT, and if it's low enough, bumping the speed limit up. But I wouldn't call that "engineering".

[Scott5114]

Caltrans can post a speed of up to 65 mph but they have to do an engineering study first.

What does an engineering study study? Do they have rules of thumb based on lots of statistics that predict accident rates from measurable road characteristics like curve radius, banking, or passing zone length? I would think it would be more about looking at the ratio between existing accident rates and AADT, and if it's low enough, bumping the speed limit up. But I wouldn't call that "engineering".

One factor you don't mention is measuring the speed that vehicles are already traveling. If the speed limit is 55 but traffic is typically flowing at 75, that is a pretty good indicator that the limit is underposted. Typically the goal is, barring any reason not to, to set the speed limit at the 85th percentile speed (the speed that 85% of traffic is traveling at or under), as that is basically what speed the drivers have collectively deemed to be "reasonable" (i.e. it assumes that 15% of drivers are those maniacs who drive 120 mph weaving in and out of lanes, and drops them out).

[Quillz]

I think a lot of speed limits are underposted, based on traffic flow. The 101 through SF Valley is officially 55-60, yet even going 70 you're often getting passed. I wonder how many of the speed limits are a relic from the 70s oil crisis and thus set to 55. 

Speaking of the 101, one thing I did notice was a maximum speed sign was recently changed from 55 to 65. But it's still pointless because no one pays attention to the maximum speed and still goes well above 65.

[FredAkbar]

I feel like the speed limit on interstates (and similar, like 101) is 55 in the major urban areas quite often. Probably due to the increased amount of onramps, lane changes, etc. But ironically, these urban areas are where you're less likely to get a ticket for going 80+, compared to rural stretches where CHP loves to set up camp and ticket people.

[SP Cook]

This is something that has wracked my brain for years and I have never found a good answer. 

The answer to all of your questions is money.  All traffic enforcement is about $$.

[Max Rockatansky]

This is something that has wracked my brain for years and I have never found a good answer. 

The answer to all of your questions is money.  All traffic enforcement is about $$.

One problem with your assessment, no law enforcement agency locally seems to give fuck all about speeding.  CHP especially only seems to care about big speeding tickets on mostly on expressways and freeways. 

[roadfro]

Caltrans can post a speed of up to 65 mph but they have to do an engineering study first.

What does an engineering study study? Do they have rules of thumb based on lots of statistics that predict accident rates from measurable road characteristics like curve radius, banking, or passing zone length? I would think it would be more about looking at the ratio between existing accident rates and AADT, and if it's low enough, bumping the speed limit up. But I wouldn't call that "engineering".

Other factors that likely play into this might include things like what the roadway grades are, whether shoulders or a decent clear zone exists (and whether the shoulders are paved and how wide they are), amount of intersections or cross traffic in a given stretch, incidents of wildlife crossing, are there private driveways that access the highway directly, among many other things... 

[pderocco]

Caltrans can post a speed of up to 65 mph but they have to do an engineering study first.

What does an engineering study study? Do they have rules of thumb based on lots of statistics that predict accident rates from measurable road characteristics like curve radius, banking, or passing zone length? I would think it would be more about looking at the ratio between existing accident rates and AADT, and if it's low enough, bumping the speed limit up. But I wouldn't call that "engineering".

Other factors that likely play into this might include things like what the roadway grades are, whether shoulders or a decent clear zone exists (and whether the shoulders are paved and how wide they are), amount of intersections or cross traffic in a given stretch, incidents of wildlife crossing, are there private driveways that access the highway directly, among many other things...

I'm just curious why it would be hard to raise the speed limit. All those things factor into the safety, but this isn't about predicting the safety of a new road. I would think it would just be a matter of looking at the stats for recent years.

[ElishaGOtis]

Caltrans can post a speed of up to 65 mph but they have to do an engineering study first.

What does an engineering study study? Do they have rules of thumb based on lots of statistics that predict accident rates from measurable road characteristics like curve radius, banking, or passing zone length? I would think it would be more about looking at the ratio between existing accident rates and AADT, and if it's low enough, bumping the speed limit up. But I wouldn't call that "engineering".

Other factors that likely play into this might include things like what the roadway grades are, whether shoulders or a decent clear zone exists (and whether the shoulders are paved and how wide they are), amount of intersections or cross traffic in a given stretch, incidents of wildlife crossing, are there private driveways that access the highway directly, among many other things...

I'm just curious why it would be hard to raise the speed limit. All those things factor into the safety, but this isn't about predicting the safety of a new road. I would think it would just be a matter of looking at the stats for recent  years.

In many locations (CA included), a speed limit isn't changed unless someone requests it be reviewed for a change. Often times this is a citizen or a local government. Who knows, why not one of us put in a request to put 60 or 65 on some random 2-lane roadway and see what happens? 

[roadfro]

Caltrans can post a speed of up to 65 mph but they have to do an engineering study first.

What does an engineering study study? Do they have rules of thumb based on lots of statistics that predict accident rates from measurable road characteristics like curve radius, banking, or passing zone length? I would think it would be more about looking at the ratio between existing accident rates and AADT, and if it's low enough, bumping the speed limit up. But I wouldn't call that "engineering".

Other factors that likely play into this might include things like what the roadway grades are, whether shoulders or a decent clear zone exists (and whether the shoulders are paved and how wide they are), amount of intersections or cross traffic in a given stretch, incidents of wildlife crossing, are there private driveways that access the highway directly, among many other things...

I'm just curious why it would be hard to raise the speed limit. All those things factor into the safety, but this isn't about predicting the safety of a new road. I would think it would just be a matter of looking at the stats for recent years.

Safety factors of a new or existing road is definitely a contributing factor in a speed study determining whether increasing a speed limit is warranted, in addition to things like AADT, previous crash data, roadway design criteria used, etc.

Say you have a two lane highway signed at 35 with no crash history, but it's winding rural route with multiple residential driveways. Safety factors such as number of driveways, sight distance, etc. might play a significant factor in a study that could recommend against an increased speed limit—in this example, reduced sight distances and increased stopping distances at higher speeds when cars unexpectedly pull out of a driveway behind a blind curve might be a significant safety concern. 

[Voyager]

One of the more interesting ones is the fact that highway 49/120 near Lake Don Pedro is signed at 65mph. Could this be a remnant from when it was going to be a freeway alignment through Priest Grade?

[Max Rockatansky]

One of the more interesting ones is the fact that highway 49/120 near Lake Don Pedro is signed at 65mph. Could this be a remnant from when it was going to be a freeway alignment through Priest Grade?

Yes, currently the two lanes are expressway grade.  You can see where the extra two lanes were supposed to go on the bridge over Don Pedro Lake.