In The American West: A New Interpretive History by Robert V. Hine, John Mack Faragher, they quote the annual homicide rate of Dodge City (one of the towns mentioned by Ridley) as 50 per 100,000 people. That would make it higher than any modern day US city except New Orleans and Detroit.
The book Homicide: A Sociological Explanation by Leonard Beeghley gives a table of homicide rates for cattle towns in the period. Abilene, Wichita and Dodge had homicide rates of 76, 91 and 160 (per 100,000 residents) respectively, much higher than any modern day US city. The Bodie Daily Free Press reported in 1881 "Bodie is becoming a quiet summer resort - no-one was killed here last week"
The book Rethinking Homicide by Meithe, Regoeczi and Drass quote McKenna 1997 as saying that Abilene had a murder rate of about 80 per 100,000, Dodge City peaked at 160, and Bodie CA at around 116.
This is not incompatible with the statement of 1.5 murders per town - the population of Dodge City in 1876 was 1,200, meaning that 1.5 murders would be the equivalent of a 125 homicide rate per hundred thousand population. However it does mean that the conclusion that this figure is "lower than the murder rate today" is completely false. A homicide rate of 125 would be much higher than any city in the modern US.
This level was not universal, however, Douglas County, Nebraska (where Omaha City is located) reported a homicide rate of 3.7 per 100,000 in the 1880s, similar to the homicide rate of northern towns of the time and low for modern US cities.. It is conceivable that some towns in the 'Wild West' had low murder rates - but that was not the norm for the cattle towns.
Research on the subject turns up a number of anti-gun-control websites claiming a low level of violence in the West. One of the most widely quoted makes this claim:
In Abilene, Ellsworth, Wichita, Dodge City, and Caldwell, for the years from 1870 to 1885, there were only 45 total homicides. This equates to a rate of approximately 1 murder per 100,000 residents per year. In Abilene, supposedly one of the wildest of the cow towns, not a single person was killed in 1869 or 1870.
Most of the others quote the exact same wording. The single source for all of these seems to be the book Frontier Violence: Another Look by W. Eugene Hollon. I have not been able to retrieve the entire text of this book to check the references. However the two claims are not consistent. For 45 killings over 15 years to give a murder rate of 1 per 100,000 this would imply that the total population of the five towns was more than 300,000. This is obviously not true. Dodge City had a population of around 1,200 and Abilene had a population of around 500. The entire state of Kansas had a population of only slightly more than 300,000 in 1870. And of course a figure of 1 per 100,000 is massively contradicted by other sources.
For Abilene, in any year in which at least one killing took place (presumably most of them, since the years with no killings are specifically mentioned) Abilene would have had a murder rate per 100,000 of 190 - again much higher than any modern US city. It is also recorded that Tom 'Bear River' Smith, Marshal of Abilene, was killed ten miles outside Abilene in November 1870 (in a year in which Libertarian Standard says no killings took place in Abilene), meaning that the definitions of "killings in Abilene" must have been taken very narrowly to reach these figures.
An interesting quote in Meithe et. al. Is that "most killings involved drunken brawls among willing participants, and public tolerance was indicated by the widespread acceptance of self defence as justification. "