Analysis of the
Neponset Greenway's Granite Avenue Crossing

Peter G. Furth
Prof. of Transportation Engineering, Northeastern University

September 28, 2000


The crossing should meet the four combined objectives of the Greenway and of Granite Av. The objectives of the Greenway are to provide a (1) safe and (2) attractive (high quality) path for cyclists and peds. The objectives for Granite Ave. are to provide (1) safe and (2) efficient passage for general traffic.

Traffic Conditions

Granite Ave. carries over 1000 veh/hr in the peak direction in weekday peak periods. Northbound queues from Hilltop St. reach back over the Greenway crossing regularly during a.m. and p.m. peaks. It is common for one to have to wait over 2 min for a gap to enter traffic from driveways next to the Greenway crossing. Weekend midday traffic is also rather heavy. Southbound traffic leaving the city has an 85-percentile speed of about 30 mph, but northbound traffic, coming from a 40 mph zone on Granite Ave. in Milton or from the expressway, probably has an 85-percentile speed around 35 mph. When there are no queues, northbound cars frequently pass at speeds over 45 mph.

When the drawbridge is up, large queues form.


There are 3 alternatives for crossing Granite Ave.:

  1. make Greenway users use the (already signalized) Hilltop St. crossing
  2. an unsignalized crossing
  3. a signalized crossing

Alternative A fails to meet both Greenway objectives. Such detours have no place in an attractive path except in extreme conditions (e.g., an overpass would be required). It would not be safe, as many path users would try to cross directly anyway. From the perspective of Granite Ave. traffic, it would also interfere more with traffic operations than a direct crossing, because it would add a conflicting movement to what is already the primary bottleneck at the Granite Ave. / Hilltop St. intersection.

Alternative B could be safe and attractive if (a) cyclists would dismount and become peds, and (2) traffic would immediately stop to let them cross. However, both premises are unrealistic.

First, many cyclists will try to ride across. Cyclists who stay on their bikes to wait for a gap in traffic would often have to wait over 2 minutes (I know from experience; a traffic study would easily confirm this). Many of them will get frustrated and attempt a dangerous crossing. Watch Shlager's trucks: the way they trucks exit, if going left to cross the drawbridge, is to fight their way through the traffic, inching out one lane at a time. That may be safe for a dump truck, but not for bikes and peds.

Second, many cars won't simply stop for peds or dismounted cyclists. With the distraction of the drawbridge and the high speed of traffic leaving the expressway, northbound traffic won't even notice the bike path and crosswalk until it's too late. And if Greenway demand exceeds one bike or ped per minute, which it certainly will, it would wreak havoc with the traffic if they did stop for all the bikes and peds.

Alternative C can be safe and attractive to bikes and peds if the signal cycle is short (and the current cycle at Hilltop St. is short enough). A ped crossing will need 17 seconds (7-second walk, 10-second flashing don't walk). Bikes need even less. For a cycle of around 68 seconds, that gives Granite Ave. green 75% of the time--no capacity problem here. So this alternative can be safe and efficient for Granite Ave., satisfying all four of the original objectives.

The signal should be actuated so that Granite Ave. is interrupted only when there's demand on the Greenway. A pushbutton shared by bikes and peds will suffice. Bicycle detectors aren't necessary, but they can add further efficiency for both bikes and the Granite Ave. traffic if used intelligently in programming the traffic control.

The traffic signal should be coordinated with Hilltop St. so that the Greenway green coincides, more or less, with the Hilltop St. green. That way the signal won't impede operations at the Hilltop St. intersection. It should also be coordinated with the drawbridge, giving the Greenway a steady green when the drawbridge is up. It should progressively stop traffic on Granite Ave. during the opening operation so that the southbound queue doesn't block the crossing, and so that the last few northbound cars that cross the bridge aren't trapped at the crossing. And while the queue is discharging just after the bridge is closed, the cycle should be doubled (making Greenway users wait twice as long) to facilitate dissipation of the queue.

Bikes should have their own signal heads. Bikes only need 3-second yellow (as at any other intersection), plus 1-second all-red is desirable. Expecting bikes to follow the ped signal is unsafe, because the 10-second flashing don't walk period presents them with a dilemma (Q: Is it safe to cross during the flashing don't walk or not? A: during the first 6 seconds, it's safe; after that it isn't.). Furthermore, because bikes needs only a 4-second change interval (yellow + all-red), during the 17-second needed for a ped crossing, they should get 13 seconds of green. Assuming a 70-second cycle, the maximum delay to bikes would be 57 seconds, which is tolerable, and the average delay to bikes would be 23 seconds, which is in keeping with the goal of making the Greenway path attractive.

Having loop detectors for bikes could improve efficiency for both Greenway and Granite Ave. traffic, if the signal control is designed to take advantage of them. Detectors can be used to extend the green as additional bikes arrive, reducing delay to cyclists and reducing the number of interruptions to Granite Ave. traffic. They can also be used to distinguish a bike actuation from a ped actuation, giving green to bikes and not to peds when there is no ped actuation, so that the interruption to Granite Ave. would only have to be 11 seconds rather than 17 seconds.


There is only one acceptable alternative for the Granite Ave. crossing: an actuated traffic signal, coordinated with Hilltop St. and the drawbridge, with both bicycle heads and pedestrian walk / don't walk heads. A pedestrian pushbutton, which bikes could share, is necessary. Bicycle detectors could also be helpful, depending on the traffic control strategies used.