Travel Route 66 today and you will find much of the route parallels the interstates that replaced it like I-57, 44, or 40. As a matter of fact you will find that some of Old 66 in its two or four lane forms is actually part of the interstate. There are examples of this in many spots.
But as a mental exercise when you do find yourself on an original section with an interstate next to it, take a few moments to conceptualize something. Take all that traffic flowing down the interstate in both directions and place it on that two lane section of 66 your traveling on. If you are nearing a large town or city its a pretty frightening thought to be stuck on a two lane highway with that much traffic, unable to find places to pass.
Well that was how Route 66 was in many places, and yes traffic was that bad even then. As matter of fact some of Route 66's sister US routes that are still active highways today, are often congested that badly. One excellent example is US 12 through Richmond, IL which on summer weekends is jammed with travelers, trucks, and locals often times for miles outside of the town.
This regular over crowding on 66 often had tragic consequences, especially as we factor in primitive traffic control devices, vehicles with few safety features, and the narrow and winding road construction methods indicative of the era in which 66 was in its heyday.
An added problem that made the highway dangerous lies within one of its nicknames "America's Mainstreet". Route 66 connected small town to small town usually via their respective main streets. This meant that there was a dangerous mixing of locals, fatigued truck drivers, and travelers in a hurry coming together amongst a tangle of intersections, side streets, and pedestrians in or near every towns center.
Eventually Route 66 began to be known as Bloody Route 66, Bloody 66, of other gruesome nicknames like Blood Alley. Know one knows precisely when theses term where first used or who first used them but the most common belief is that it was first coined in the late 40's by Illinois State Troopers for a section of Route 66 between Dwight and Lincoln, IL.
But Illinois State Troopers weren't the only ones to use this term and have sections of road like it. Perhaps with the exception of Kansas which only had a few miles of 66 running through it, just about all the other states had sections of highway that routinely saw the horrors motor vehicle accidents. As a matter of fact in 1956 alone Arizona State Police estimated that 1 of every 6 traffic fatalities in Arizona occurred on Route 66. Other estimates say that from Missouri to Texas nearly 4000 people lost there lives on the route in a 20 year span between 1935 and 1955.
In Jon Robinson's book "Route 66: Lives on the Road", the author interviews several state police officers. For the most part they give a lighthearted account of their careers along the route, but can't help not alluding to traffic accidents they had worked over the years.
Obviously thanks to the interstates, safer cars, and modern traffic control the Route 66 you can travel now is far safer then it ever has been. But knowing this bit of history about 66 is a good glimpse into why the highway had to be replaced by the Interstates. It's also great insight as to why so many towns had to be bypassed leaving behind relics of what once was a bustling industry of tourist services.