高速公路入口的匝道信号灯每隔几秒才放行一辆车，为什么这样做反而能让整条走廊更畅通？
Freeway on-ramp signals release only one vehicle every few seconds—why does holding traffic back leave the whole corridor moving better?
主线车流密度一旦逼近临界值，微小扰动即可触发流态失稳，车速与通行能力同时骤降，形成走走停停的激波。匝道信号把汇入率限制在与主线剩余通行能力匹配的水平，多余车辆暂时在匝道上排队，使主线始终处于临界密度以下的稳定区；拉大的汇入间隔也免去了主线车辆频繁减速避让。排队延误小于主线提速带来的收益，断面总通过量不降反升。
Once mainline density nears its critical value, a tiny disturbance triggers flow breakdown: speeds and throughput collapse together into stop-and-go shockwaves. Ramp metering holds entering traffic to match the mainline's residual capacity, queuing surplus vehicles on the ramp so the mainline stays in the stable regime below critical density. Wider merge gaps also spare through drivers constant braking. Ramp delay costs less than the benefit of keeping the mainline fast, so total throughput rises.
难度 进阶 · 适合 大众
CC BY 4.0 · 来源待核验 · Source pending review
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出处备注：著录含书名，但缺版次 / 出版社 / 年份中的一部分，需编辑补全后才能做独立比对。
M.帕帕乔治欧与A.科齐亚洛斯《高速公路匝道控制综述》，IEEE智能交通系统汇刊，第3卷第4期，2002年。
M. Papageorgiou & A. Kotsialos, Freeway Ramp Metering: An Overview, IEEE Transactions on Intelligent Transportation Systems, vol. 3, no. 4, 2002.
独立核验依据 · Independent verification: 核验依据 1 CROSSREF 标识 10.1109/tits.2002.994814：IEEE Transactions o · 核验依据 3 Crossref DOI 10.1109/tits.2002.806803
#匝道控制 #交通流失稳 #通行能力
信号交叉口的周期是不是越短越好？Webster 最佳周期如何在两种损失之间权衡？
Is a shorter signal cycle always better at an intersection? How does Webster's optimum cycle time balance two kinds of loss?
周期太短，每个周期都要重复支付起步损失时间，绿灯还容易放不完排队；周期太长则红灯等待被拉长，行人过街也更难安排。Webster 在1958年的信号配时研究中给出使平均延误最小的最佳周期 C₀=(1.5L+5)/(1−Y)，其中 L 为每周期总损失时间，Y 为各相位流量比之和。可见周期宜随交通负荷增大而适当延长，但不能过长。
With too short a cycle every phase pays start-up lost time again and queues risk not clearing on green; with too long a cycle red waits stretch out and pedestrian stages become hard to fit. From his 1958 signal timing study, Webster derived the optimum cycle C₀=(1.5L+5)/(1−Y) that minimizes average delay, where L is total lost time per cycle and Y sums the critical flow ratios of the phases. Cycle length should therefore grow moderately with traffic demand, but not without limit.
难度 进阶 · 适合 初中(12-15)
CC BY 4.0 · 来源待核验 · Source pending review
来源 · Source
出处备注：著录含书名，但缺版次 / 出版社 / 年份中的一部分，需编辑补全后才能做独立比对。
F.V.韦伯斯特《交通信号设置》，英国道路研究所技术论文第39号，HMSO，伦敦，1958年。
F.V. Webster, Traffic Signal Settings, Road Research Technical Paper No. 39, HMSO, London, 1958.
#信号配时 #Webster公式 #交叉口延误
与信号灯控制的交叉口相比，环形交叉口（环岛）为什么通常能减少死伤事故？冲突点数量和车速各起什么作用？
Compared with signal-controlled intersections, why do roundabouts usually reduce fatal and injury crashes, and what roles do the number of conflict points and vehicle speeds play?
四路信号交叉口存在约32个交叉、合流冲突点，直角碰撞与左转对撞的冲击速度高；环岛把冲突点减至约8个且均为低角度合流分流，车辆进入前必须减速让行，环内车速一般低于30公里/小时。碰撞角度变浅、速度降低使事故动能大幅下降，死伤事故随之明显减少；但流量过大时环岛易拥堵，需按交通量选型。
A four-leg signalized intersection concentrates about 32 crossing and merging conflict points, and right-angle and left-turn opposing collisions occur at high closing speeds. A roundabout cuts this to roughly 8 low-angle merging and diverging conflicts and requires entering drivers to yield and slow down, keeping circulating speeds below about 30 km/h. Shallower impact angles and lower speeds dissipate most crash energy, so fatal and injury crashes drop markedly. The tradeoff is capacity: under very heavy demand roundabouts can congest, so the design must match the traffic volume.
难度 进阶 · 适合 大众
CC BY 4.0 · 来源待核验 · Source pending review
来源 · Source
出处备注：著录含书名，但缺版次 / 出版社 / 年份中的一部分，需编辑补全后才能做独立比对。
B. W. Robinson 等《Roundabouts: An Informational Guide》（FHWA-RD-00-067），美国联邦公路管理局，2000年
B. W. Robinson et al., Roundabouts: An Informational Guide, FHWA-RD-00-067, Federal Highway Administration, 2000
#环形交叉口 #冲突点 #交通安全 #减速让行