Editing Interstate 19 (section)

==Route description==
[[File:I-19 southern terminus.jpg|thumb|left|I-19 becomes an at-grade divided freeway just west of its southern terminus in Nogales. The US–Mexico border is just south of the Interstate (left of image).]]
In [[Nogales, Arizona|Nogales]], the southern terminus of I-19 is at West Crawford Street, adjacent to the international port of entry, and southbound travelers can continue into [[Heroica Nogales, Mexico]], via state-maintained surface roads, and connect with [[Mexican Federal Highway&nbsp;15|Federal Highway&nbsp;15]] (Fed.&nbsp;15) either to the south or west of Nogales, Sonora.<ref name=google>{{google maps |url = https://maps.google.com/maps?f=d&hl=en&geocode=15839470934785341138,31.334684,-110.941493&saddr=W+Crawford+St%2FI-19+%4031.334684,+-110.941493&daddr=32.198712,-110.978565&mra=mi&mrsp=1,0&sz=16&sll=32.196678,-110.978479&sspn=0.008207,0.01442&ie=UTF8&ll=31.814563,-111.011353&spn=1.054928,1.845703&z=9 |title = Overview Map of I-19 |access-date = February 15, 2008 }}</ref>

Starting from the southern terminus at kilometer post&nbsp;0 ([[#Signage|not milepost&nbsp;0]]), I-19 initially heads briefly south then west on surface streets, navigating its way through the town of Nogales for {{convert|0.2|mi|km|sp=us|order=flip}} before becoming an [[Interstate Highway standards|Interstate-grade freeway]] and making the turn to head north toward Tucson. It has interchanges with two other state highways near the southern end of the route, [[Arizona State Route 189|State Route&nbsp;189]] (SR&nbsp;189) at exit&nbsp;4 and [[Arizona State Route 289|SR&nbsp;289]] at exit&nbsp;12. The interchange with SR&nbsp;189 at exit&nbsp;4 both serves to funnel traffic so as to bypass around Nogales and Heroica Nogales for travelers bound to or from [[Hermosillo]] or [[Mexico City]] and provides for the continuous flow of freight and truck traffic through the larger [[Nogales-Mariposa Port of Entry]] to Fed.&nbsp;15, which has its northern terminus at the US–Mexico border with SR&nbsp;189 and its southern terminus {{convert|2179|km|mi|sp=us}} away in Mexico City.<ref>{{cite map |url = http://www.viamichelin.co.uk |title = Calculate your route |publisher = Michelin |access-date = August 11, 2010 }}<!--End points were Nogales and Cuidad de México; kilometers were chosen as this road is in Mexico--></ref> After exiting Nogales to the north, I-19 passes near and around a series of sparsely-populated towns and retirement communities along the banks of the [[Santa Cruz River (Arizona)|Santa Cruz River]], including [[Rio Rico, Arizona|Rio Rico]], [[Tubac, Arizona|Tubac]], [[Amado, Arizona|Amado]], [[Green Valley, Arizona|Green Valley]], and [[Sahuarita, Arizona|Sahuarita]]. For several miles near Amado and Green Valley, the eastward view from I-19 provides scenic views of [[Madera Canyon (Arizona)|Madera Canyon]] and the [[Santa Rita Mountains]] in the [[Coronado National Forest]].<ref name=google/>

[[File:I-19 northbound at I-10 junction.jpg|thumb|I-19 northbound at the I-10 interchange; the downtown Tucson skyline can be seen on the left.]]
Just before entering [[Tucson, Arizona|Tucson]], I-19 passes through the eastern section of the [[San Xavier Indian Reservation]] where it makes its only crossing of the Santa Cruz River. As I-19 enters the Tucson city limits, it has an interchange with [[Arizona State Route 86|SR&nbsp;86]] at exit&nbsp;99 before reaching its northern terminus at an interchange with I-10.<ref name=google/>

Nearly the entire route of I-19 follows, or is adjacent to, the former routing of [[U.S. Route 89 in Arizona|US Route&nbsp;89]] (US&nbsp;89) and the Santa Cruz River, which flows northward from Mexico, through Tucson and usually disperses into the desert between [[Marana, Arizona|Marana]] and the [[Gila River]], southeast of [[Phoenix, Arizona|Phoenix]]. Most of the time, much of the river is dry, but heavy storms can cause it to overflow its banks, flooding farmland before reaching the Gila River.

===Signage===
{{more citations needed|section|date=May 2020}}
{{multiple image
 | align = right
 | direction = vertical
 | width = 250
 | header = I-19 Signage
 | image1 = Metric Interstate 19.jpg
 | alt1 = Metric-unit advance guide sign on I-19, installed as part of the Valencia Road interchange renovation
 | caption1 = Metric-unit advance guide sign on I-19, installed as part of the Valencia Road interchange renovation
 | image2 = Metric Signs In Use On Interstate 19.svg
 | alt2 = Advance Guidance Sign alerting Drivers of I-19's Metric Signage located near Exit 99 (Southbound) for West Ajo Way [[Arizona State Route 86|SR 86]]
 | caption2 = Advance Guidance Sign alerting Drivers of I-19's Metric Signage located near Exit 99 (Southbound) for West Ajo Way [[Arizona State Route 86|SR 86]]
}}
I-19 is unique among US Interstates because signed distances are given in meters (hundreds or thousands as distance-to-exit indications) or kilometers (as distance-to-destination indications), and not miles. However, the speed limit signs give speeds in miles per hour. According to the [[Arizona Department of Transportation]] (ADOT), metric signs were originally placed because of the [[Metrication in the United States|push toward the metric system]] in the US at the time of the original construction of the highway.<ref>{{cite news |first = Denise |last = Holley |date = May 14, 2009 |title = ADOT Defends Replacing Metric Signs along I-19 |url = http://www.nogalesinternational.com/news/adot-defends-replacing-metric-signs-along-i/article_47be9930-829e-579c-8c88-a706dee9cd40.html |work = Nogales International |access-date = April 23, 2014 }}</ref>

I-19 had originally been signed as it was constructed, in a series of small signing contracts that used customary units.<ref>One example of such a contract was Arizona federal-aid project I-19-1(65), covering I-19 between its Nogales terminus and the Otero TI (title sheet signed August 7, 1970).</ref> In 1980, ADOT awarded a single contract to install new signs which used metric units, to overlay customary-unit expressions on some existing signs with metric-unit expressions, to install kilometer posts, and to provide bilingual signing in select locations. The signing scheme used in 1980 provided explicit units on advance guide signs, but not on interchange sequence signs or post-interchange confirmation (distance) signs.

The expressions on advance guide signs were of the form "2&nbsp;km" ({{Convert|2|km|mi|disp=output only}}) for distances over {{convert|1|km|mi|sp=us|spell=in}} and "500&nbsp;m" ({{Convert|500|m|mi|disp=output only}}) for distances under {{convert|1|km|mi|sp=us|spell=in}}, with no provision for fractional kilometrages. On advance guide signs, the metric unit expressions "km" and "m" were placed on the baseline where "MILES" would otherwise have gone but were sized so that their lowercase loop height matched the uppercase letter height of "MILES" on customary-unit signs.

The exception was a handful of advance guide signs for the SR&nbsp;86 (Ajo Way) exit, which used "KM" (in uppercase, contrary to [[International System of Units|SI]] nomenclature) on the same baseline and at the same letter height as "MILES". The 1980 signing plans also provided design details for speed limit and advisory speed signs using metric units, with the limit values enclosed in a red circle on the speed limit sign and a black circle on the yellow-background advisory signs. These speed signs all had explicit units, with "km/h" below the circle enclosing the limit value. However, the advisory signs were canceled by change order and not installed.

Had the metric speed limit sign been installed, the signed speed limit on I-19 would have been 88 km/h, which is a close soft-conversion of the then-existing {{convert|55|mph|km/h|sigfig=4|abbr=on}} [[National Maximum Speed Law|national maximum speed limit]]. As Arizona's current maximum speed limit is 75 mph, the metric equivalent would most likely read {{convert|120|km/h|mph|sigfig=4|abbr=on}}. Information signs, to three distinct designs, were also placed at various locations on or near I-19 to advise motorists that the highway was signed in metric. Notwithstanding the metric legends, the signing plans were dimensioned entirely in feet and inches.<ref>Construction plans for Arizona federal-aid project I-19-1(81) (title sheet signed January 17, 1980).</ref>

In 1999, ADOT awarded two contracts (administered as a single construction project) to renew the signs along the full length of I-19. The general approach toward metric signing differed from that taken in 1980. Explicit units were given not just on advance guide signs, but also on interchange sequence signs, post-interchange confirmation signs, and community interchange signs (the last-listed had not been used in 1980). On the distance signs, "km" appeared after each kilometer measurement except when one or more of the distances was a fractional kilometer.

In such cases, all the distances were given in meters with "meters" (written out in full, not "m") after each distance value. On distance signs in general, "km" or "meters" appeared on the same baseline and with the same letter height as the distance values, while advance guide signs were formatted as in 1980. Since a typical ADOT freeway guide sign rehabilitation contract also replaces surface road signing near those roads' interchanges with the freeway, metric-unit signs also appeared on local roads near I-19, giving distances in kilometers to tourist attractions such as [[Mission San Xavier del Bac]].

As was the case in 1980, the signing plans were dimensioned in feet and inches.<ref>Construction plans for Arizona federal-aid projects NH-19-1(110) (ADOT TRACS H260701C) and NH-19-1(116) (TRACS H260702C) (most sheets sealed March 1997).</ref> However, a number of signs near the Valencia Road interchange were replaced or amended when it was converted from a [[partial cloverleaf interchange]] to a [[single-point urban interchange]] in 2000. One of these signs has a fractional kilometrage greater than {{convert|1|km|mi|sp=us|spell=in}}, rendered as "1500&nbsp;m" ({{Convert|1500|m|mi|disp=output only}}), while others use "m" rather than "meters" as the unit expression. Metric-unit expressions on the advance guide signs installed or modified as part of this contract appear on the same baseline as the metric values, rather than on a raised baseline as on other I-19 advance guide signs. Again, the plans were dimensioned in feet and inches.<ref>Construction plans for Arizona federal-aid project ACNH-19-1(127) (TRACS H260901C) (most sheets sealed June 1999).</ref>

Citing motorist confusion arising from the metric signs on I-19, ADOT's Tucson district announced that new signs on I-19 would use [[US customary units]]. To avoid the cost of replacing the metric signs all at once, signs would be replaced in specific areas of the freeway during construction projects in those areas.<ref>{{cite news |first = Susanna |last = Cañizo |date = January 19, 2004 |url = http://www.azstarnet.com/dailystar/printDS/6429.php |title = Some I-19 Metric Signs Going |work = [[Arizona Daily Star]] |location = Tucson |archive-url = https://web.archive.org/web/20050223220749/http://www.azstarnet.com/dailystar/printDS/6429.php |archive-date = February 23, 2005 |access-date = June 13, 2012 }}</ref> New signs were put into place between exit&nbsp;99 (Ajo Way) and exit&nbsp;101 (I-10) in 2004 after the completion of the new I-10/I-19 interchange.

{{As of|2010}}, the remainder of the project has been stalled due to local opposition, particularly from businesses that would have to change their directions.<ref name="local opposition NYT story">{{cite news |last = Lacey |first = Mark |title = Metric Interstate Divides Arizonans |url = https://www.nytimes.com/2010/09/15/us/15highway.html?hpw |newspaper = [[The New York Times]] |date = September 14, 2010 |access-date = September 14, 2010 }}</ref>

A reconstruction project at the Interstate's northern terminus with I-10 in Tucson (at the interchange commonly called the Crossing) began in 2002 and was completed in August 2004.<ref>{{cite web |author = Staff |url = http://www.1019crossing.com/ |title = Home Page |work = The Crossing: I-10/I-19 Interchange |publisher = Arizona Department of Transportation |archive-url = https://web.archive.org/web/20050207012249/http://1019crossing.com/ |archive-date = February 7, 2005 |access-date = November 27, 2011 }}</ref>