Quite a lot as it turns out. Does your street have just four travel lanes and sidewalks? A design pulled from the golden era of the automobile? If it does it's likely around 20 metres, a pretty common right-of-way width for streets. How a 20 metre wide street can look and function can be very differently depending on your priorities. Read on for many different examples and some thoughts on the pros and cons of each.
First up, lets start with the four laner and sidewalks. Chances are, if turns are allowed from the left lane in each direction, the actual through capacity is little more than that of a single lane. Drivers likely avoid the left lane unless they're turning. The right lanes may be parking lanes during off-peak times effectively reducing the street to two lanes. Pedestrians do ok, although walking so close to four lanes of traffic with little separation other than a curb is never pleasant. And cyclists, you get nothing, maybe a token Sharrow to remind you that you don't have a dedicated space of your own.
We can do much better, and provide much greater transportation equity for all in the community by providing adequate infrastructure for all modes. Despite drivers feeling hard done by when bike lanes are introduced, they often don't appreciate how privileged they are, especially compared to those that can't or don't want to own a car. If we know those two left lanes in either direction are rarely utilized due to turning traffic, we can take those two lanes and turn them into one turn lane for either direction. This, along with some lane narrowing, creates sufficient space to provide safe bike facilities on either side of the three vehicle lanes, and even a buffer. A road diet in other words. This can be done very cheaply without the need for reconstruction, simply change the paint, maybe provide some form of physical separation such as planters or delineator posts, depending on the space available.
As an alternative if space is tight, a bi-directional protected bike lane may work better. This is less preferred than uni-directional lanes due to the somewhat unconventional movements relative to traffic. In the image below, imagine a driver heading to the right and turning left, he can see the oncoming cyclist quite easily as he's travelling in the same direction as opposing traffic, but the cyclists heading to the right could approach from outside his line of sight as he's about to make his left turn. This is more problematic where cyclists speeds are higher and the likelihood of "appearing out of nowhere" is higher. The bi-directional option generally requires less space, so subject to exact dimensions of the existing roadway, it may be more feasible than uni-directional, but care must be taken at intersections.
Now what if we had the funds to do more with the street? To change it completely, rip out the curb and gutter and start over? Perhaps this street is being revitalized, new shops are appearing, there is a need to slow traffic down, maybe reduce volumes, provide storefront parking, what else could we do there? Narrow the lanes? Widen the sidewalk, provide curbside parking, add a boulevard to separate pedestrians and/or cyclists from vehicles? At intersections, if we have parking we can restrict it and replace it with a centre turn lane. Cyclists are still on the roadway sharing it with vehicles, not ideal, but hopefully the speed and volume has reduced from the four laner we started with.
A variation on the above, somewhat similar to the Amarture in Edmonton I just visited, would be to pave the entire street with consistent paving bricks a little change in surface elevation, creating a blur between the vehicle space and pedestrian space, slowing vehicles down further, and inviting pedestrians to cross as required.
Now those last two options, left bicycles on the street mixing with cars, cars that should be fewer, and travelling slower than before, but still, it's mixing with cars, not comfortable for many cyclists, even if speed and volume is reduced. How can we get them off the street and give them a safe space? It likely requires another use to give up space. If we were to add off-street bike paths adjacent to the sidewalk, we would provide safe cycling facilities for all ages and abilities, and still have space for some limited patio seating. The downside of this, other than loss of parking is the potential for pedestrians and cyclists to mix. Different surfacing and pavement markings can help define areas, but if pedestrian demand is high, it will likely spill over onto the bike path. If demand is lower, this can work pretty well.
If the loss of parking is too much of a deal breaker, retaining even one side of parking, plus boulevards and some patio space leaves little space for safe bike facilities. We could feasibly add a multi-use path on one side, and by mixing bikes and pedestrians we save a considerable amount of space, but at the expense of increased conflict. Reasonable for slow cycling, but not so good to have higher speed cyclists whizzing by pedestrians and patios. those faster cyclists do still have the option to stay on-street, leaving the multi-use path for the less experienced, but nevertheless, this isn't a great solution. Turn lanes can be formed by removing the parking and potentially trimming the boulevard near the intersections.
A different option is to dedicate a large amount of space to patios. If vehicles can be slowed sufficiently, and keeping cyclists on-street can be done reasonably safely. We can allocate a lot of space to pedestrians and patio's. It all depends on the vision for the street and the adjacent land uses. If the vision is a bustling street with lots of life and activity, this may make sense. There is still no parking, but is it even necessary if we have increased development nearby that provides walk-in customers to the effectively increased floor space?
Going back to the protected bicycle options, the previous off-street bike paths took cyclists far away from vehicles but put them in close proximity to pedestrians. We could have removed the patio space and put better separation between cyclist and pedestrian. Below is another variation on that but with cyclists on-street. This extra separation of course takes up space and removes the space for even small patios.
As we did at the start, going to a bi-directional facility helps us take some of that space back to retain the patios, but comes with those challenges of cyclists approaching from outside a drivers line of vision.
The final option I have is the fabled Woonerf, a Dutch street design ideology if you can call it that, that provides a shared space with a more limited vehicle path along the street. By design, it slows vehicles down closer to a pedestrian pace, greatly increasing safety for vulnerable modes, while still allowing for access and some parking. Better yet, pedestrians are free to wander about without risk of being accused of jaywalking. This is great where there is significant commercial activity and the pedestrian demand to support such a pedestrian orientated street.
The examples above are just some of the options possible with a 20 metre right-of-way. There are of course a near infinite number of variations and tweaks possible to fit a particular street and its unique needs.